9 of them are “essential” because your body can’t make them from other amino acids.
Protein is complete when it has enough of all of the essential amino acids. It’s incomplete when it’s missing at least one of them.
Rice, for example, doesn’t quite have enough lysine in it. If you live on only rice, you’ll eventually run into a lysine deficiency. Chickpeas, though, have plenty of lysine but not enough methionine.
Rice and chickpeas individually are incomplete. A bowl of chickpeas and rice is complete, though.
The problem with this, like I said, is that if you have cucumber sushi for lunch and falafel dipped in hummus for dinner it’ll average out to be complete. Almost no one has to actually care about this, it’s really just an interesting factoid.