George has promised Lennie's Aunt Clara that he will watch out for Lennie, and he does this in numerous ways, constantly, in John Steinbeck's story Of Mice and Men.
As the men approach the water hole, George tries to protect Lennie from drinking bad water. We get the sense quickly that something is not quite right with Lennie because George has to explain very simple concepts to him, and Lennis forgets a lot.
We find out, also, that George had to make a run for it with Lennie, escaping and hiding to protect Lennie, when Lennie got carried away with feeling a young girl's dress (he loves soft things) and frightens her.
So [Lennie] reaches out to feel this red dress an' the girl lets out a squawk, and that gets Lennie all mixed up, and he holds on 'cause that the only thing he can think to do...
When the girl says she was raped, the authorities come after Lennie with men ready to lynch him, and George has to hide Lennie and then make sure they get away.
George watches out for Lennie when Curley comes in and starts picking a fight. He lets it go on for a while, but then tells Lennie to stand up for himself—to fight back. Lennie has no desire to hurt anyone, but in defending himself, he eventually crushes Curley's hand. Embarrassed, Curley explains to the others on the ranch that he got it caught in some machinery.
George tells Lennie how he should behave on the job. In particular, he tells Lennie to stay away from Curley's wife. Lennie remembers this clearly and tries hard to do as he has been told, but Curley's wife will not leave Lennie alone, and ultimately tragedy follows.
George also gives Lennie a dream—in essence, George has a dream and lets Lennie share it. George wants to own a piece of land with a house, and work the land. This also becomes Lennie's dream, as George tells him that he can care for the rabbits. Lennie is overjoyed.
In the end, when the men on the ranch chase Lennie to kill him for the accidental death of Curley's wife, it is George who takes Lennie's life, knowing how horrific it would be for the sweet-natured Lennie to face the anger of a mob and death at the hands of men with murder on their minds. It is just another way that George watches out for Lennie.