The ELI5 version of this:
A pitcher is required to do certain things in a certain way when they are in the process of releasing a pitch. If they do not do them in the right order, or if they start doing them and get to a certain point but then stop, it is a balk.
That is because the runner may rely on the pitcher doing those things in order to determine when to start running on a steal attempt (or "runners going" situation, like 2 outs 3-2 count runner on first). If the pitcher does not do the things they are required to do, or starts to do them and then stops too late in the sequence, then it misleads the runner.
