Follow-on is a term used in the sport of cricket to describe a situation where the team that bats second is forced to take its second batting innings immediately after its first, because the team was not able to get close enough to the score achieved by the first team batting in the first innings. If the second team to bat scores substantially fewer runs than the first team, the first team can enforce the follow-on, instructing the second team to bat again immediately. In this case the sequence of batting innings will be first team, second team, second team and then, if needed, the first team again. This is in contrast to the normal progression of batting innings which is first team, second team, first team, second team.
This rules governing the circumstances in which the follow-on may be enforced are found in Law 13 of the Laws of cricket.
Read more about Follow-on: Minimum Lead, Enforcing The Follow-on, Victories By Sides Following-on, Longest Period Without Being Forced To Follow-on in Test Cricket, The History of The Follow-on