Don't tell lies. Like the first precept, this one is pretty clear as a moral standard. But, also like the first precept, it is problematic for people when applied too narrowly, and the mind is especially prone to crafting hypothetical scenarios in order to seemingly thwart it or disprove it.
The classic scenario goes something like this: Anne Frank is in your attic. The Gestapo stops by and asks you if you are harboring any Jews. Is it okay to lie? If it is, then what about this precept? If it is not, then this precept is bullshit. Again, this is your mind at work, doing it's thing. Just watch it.
What is at work there is your persistent dissatisfaction with not having a definite, concrete, eternal concept to which to cling. How easy it would be to just be able to decide "I will never lie" and have boundless confidence that every situation will magically work out peachy-keen! This is what your mind is in pursuit of here.
Anne Frank is actually not in your attic, and she never will be. The precepts are not rules to apply to solving abstract ethical puzzles, they are reminders for us as we make decisions about the conduct of our real, actual, everyday lives. They are not absolute, unchanging, concrete and definite. But, take a moment to notice something here--your real, actual, everyday life is not absolute, unchanging, concrete and definite.
When I have lied, I have done so because I bought into the idea that I would be better off if someone else understood the world in a different way than I knew it to be. This is the grasping, this is the delusion that this precept reminds us we can get caught by. However, there is a broader view still of this precept. Speaking deceptively can be much more subtle than outright lying, it can mean representing an opinion as fact, and this process can be entirely internal. That is, you can speak deceptively to yourself. As with all the precepts, this is where the mistake is really made, I.e., in your own mind.
I have several close friends who are couples. That is, I am friends with two people whom I met independently of each other who are now pair-bonded, either by marriage or some other similarly primary relationship. So, I can find myself in a situation where I am getting too sides of the same story, two versions of a single event. Now, I long ago decided to never step in the middle and this precept points to one of the reasons why.
Even after I have heard the accounts from both sides I don't really know what happened. I often believe I do, I often have a very concrete concept of where the misunderstanding lies, and who is at fault, and all that stuff. This precept serves me by reminding me that what I really have is a third version of the story, one step abstracted from the actual event (unless I was there) and to represent that to anyone, including myself, as the truth is to speak deceptively.
Of course, it is fine for me to discuss my view, or my opinion of a situation as long as I represent it as such. This doesn't mean that I can never talk about anything. But much of the speech in which I used to engage was some version of me trying to convince someone that the world really is as I see it. If I had limited myself to only things I knew to be true I would have really been a very quiet person all those years.
In fact, among people who have only met me recently, I actually have a reputation for being quiet. This may be because I do stop myself from simply babbling on these days some narrative being cooked up in my mind, because I see it for what it is--so much meaningless mental white noise. I endeavor to limit my speech to things I know to be true, helpful and kind. It never ceases to amaze me how little of what I want to say actually satisfies those criteria. Try it for yourself.
While this precept does represent an admonition against actual lying, and that is good advice, the real issue here is seeing what your mind does with an event as you relate it later. Whenever you speak to anyone about anything you are conceptualizing reality. You are extracting items from the whole picture, which means some things are inevitably left out.
There's nothing to do about that other than see it for what it is. You can never tell the whole truth about something. The only Truth is what is going on at this very moment, and that can't be spoken. But, as a beloved teacher in my lineage is known to have said, you have to say something or there will be no understanding. This is the double-bind we find ourselves in here, so what we can do is be vigilant to represent ourselves with speech as faithfully and truthfully as possible.
So, a follower of the Way does not speak deceptively.