Photo by Gideon.
A baseline is simply the way a person behaves normally, without any special reason to act otherwise. We often try to watch body language for things like whether someone's lying or if they're attracted to us, but not everyone behaves the same way under those circumstances. By watching for someone's baseline, you learn how they behave regularly, and then you can watch for when that behaviour changes. As tips site Barking Up the Wrong Tree explains:
If someone is always jumpy, jumpiness doesn't tell you anything. If someone is always jumpy and they suddenly stop moving — HELLO. So ask yourself: "Is this how they normally act?"
This step is so crucial to reading someone's body language that you should practice it long before you ever try to find out what a particular action "means." The next time you're interacting with someone, take a second to observe things like where they put their hands, how they position their feet, or how much they smile. You don't need to take notes (that's creepy), but simply practice being observant. The more practice you get, the easier it will be to notice changes in body language when they happen.
This Is How To Read People: 5 Secrets Backed By Research [Barking Up the Wrong Tree]