A "lag" is the period of time that you have to wait for between two things that are supposed to happen at the same time.
For example, when you speak with someone, you usually hear their words immediately as they are saying them. But when you're video chatting with them, it can take a few seconds for the signal to travel to you. So there's "a lag".
Another example of "a lag" is when you're using a computer that's really slow. If you push a button, but it takes a few seconds before anything happens, you can say:
There's an annoying lag on this computer.
You're probably familiar with the term "jet lag". Jet lag is a feeling of tiredness that you get when you travel to another time zone. You can think of it as "a lag" between the actual time in the new location and the time that your body thinks it is.