What does "peaks and valleys" mean?

Vocabulary Word: peaks and valleys

1. Definition (expr.) This week, instead of writing a definition, we chose a perfect explanation written by Jesus (Torreon Coahuila, Mexico):
“Sometimes you are up, everything is good, nice, you feel happy etc… And sometimes you are down, everything goes wrong, everthing is bad, you get desperate etc…”

Examples “My relationship with my girlfriend has peaks and valleys. Sometimes it’s really fun, but other times we argue and fight.”

Examples “The Los Angeles Lakers are going through a lot of peaks and valleys this season. One game, they are the best team in the league and the next game they are terrible.”

Examples “My job is full of peaks and valleys. Some days, it’s the best job in the world, but other days I absolutely hate it!”