The word “just” can be used in two ways:
1) Recently
Be careful – I just washed the floor, and it’s still wet.
(= I washed the floor a few minutes ago)
He just finished a big project.
(= he finished the project very recently)
2) Only
I have just one brother. (= I have only one brother)
I thought you were hungry, but you ate just half of your sandwich.
(= you ate only half of your sandwich, and no more)