The climax is the turning point of the story, or the most exciting part.
The best way to remember the climax is that everything changes. After the climax, things are so drastically different that things can never go back to the way they were. The story doesn’t end, because the characters have to deal with the aftermath of the climax and resolve the new conflicts it presents. The climax is also the high point of the action, and the most interesting or most exciting part of the story
In Of Mice and Men, the climax is when Lennie accidentally kills Curley’s wife. All of the other conflicts- George and Lennie trying to keep their job, conflicts with Curley, and Curley’s wife being lonely and flirtatious- are resolved. The one conflict remains: what will happen to Lennie?
After Lennie kills Curley’s wife, George and Lennie cannot just go on pretending nothing happens. George solves this new conflict by killing Lennie, and putting him out of his misery. It is an act of kindness, like putting down an old dog. If Lennie had lived, he most likely would have gone to prison even though he didn’t mean to do anything wrong.