What Is Follow On in Cricket

The follow-on.
In a two innings match, if the side batting second scores substantially fewer runs than the side batting first, the side that batted first can force their opponents to bat again immediately.
The side that enforced the follow-on risks not getting to bat again and thus the chance of winning. For a game of five or more days, the side batting first must be at least 200 runs ahead to enforce the follow-on; for a three- or four-day game, 150 runs; for a two-day game, 100 runs; for a one-day game, 75 runs. The length of the game is determined by the number of scheduled days play left when the game actually begins.
five or more days All team out and remain runs are more than least 200 runs
for a three- or four-day All team out and remain runs are more than least 150 runs
for a one-day game, All team out and remain runs are more than least 75 runs
Is called fellow-on
Is from Law of Cricket no 13

The simple way to define the fellow-on in test cricket is
follow in test cricket is calculate in the way that the runs scored by team B has to be less than team A by at least 200 runs less in first inn