I am not much of a baseball afficionado, but here goes my attempt at an explanation.
In baseball, the game is divided into a series of plays known as innings. Each game has nine innings (unless there is a tie, and then the game goes into extended innings).
During each inning, each team (there are two) has a chance to score, while "at bat." The other team, playing defense, tries to get the batting team out. After three "outs" the defending team then gets a chance "at bat."
The first half of each inning, while the away team is at bat, is called the "top" of the inning.
The second half of the innning, while the home team is at bat, is called the "bottom of the inning."
Innings do not have a definite time of play. An inning is over after each team has achieved three "outs." This could take a few minutes, if the opposing team's defense is very, very good. Or, it could take up to thirty-forty minutes (or even hours), if the defending team is not very good. This is why baseball games can take up to five or more hours to play.
Thank you so much to you both!
Your explanations were brilliant.
Now, I have all the elements to find the right word (what I had read on the internet was a bit complicated and hadn't really help me to find the proper equivalent).