George wants Lennie to remember where they're camping because Lennie's behavior in the past has forced George and Lennie to have to leave suddenly. Such was the case when Lennie was accused of attempted rape when he felt the skirt of a woman. He and George had to hide from the men looking for them until they could escape. George wants Lennie to know where he should go if he gets into trouble again.
George tells Lennie to do this so that if, for any reason, Lennie gets in trouble, he can remember how to get back to this place and hide.
I believe that this passage is here to act as a bit of foreshadowing. Later in the book, Lennie will get in very serious trouble when he accidentally kills Curley's wife. When that happens, he will come back here to where the two of them are camped. He will hide in the brush until George comes, just like George is telling him to at this point early in the book.
In the novel 'Of Mice and Men' by John Steinbeck, the author depicts Lennie as a character who has learning difficulties. He and his friend/protector George are able to get work because of Lennie's strength, but this blessing has also been a curse. It has gotten them into trouble when Lennie, not knowing his own strength, goes over social boundaries and scares people (girls) becuase he doe not know his own strength, In case he needs to run away from such trouble again, George has to drive home again and again the directions to a 'safe place; - what it looks like, how to get there, what to do next etc. Learning challenged Lennie needs lots of repetition and reinforcement so he will remember. Bad short term memory is part of his condition.
In the short book "Of Mice and Men" George and Lennie are arriving at a place just off of the road near a river bank. George and Lennie had to leave the last place of their employment because George had seen a girl wearing a pretty dress that he had liked. He had grabbed at the dress which startled the girl. The people had come after them to beat Lennie up and they had o escape from the situation. Lennie is pretty innocent but does not understand social boundaries which gets him into trouble at each job site. George is trying to set up a safety net in-case it happens again.
This part of the story also sets the reader up to see what will come in the suture and to give a clue that there is a need for them to have a safe place. George later tells Slim the story about the girl and how they had to hide a whole day in the water so they would not get caught.
Because if lennie ever does anything bad george will be able to find lennie before anyone else does and does something without george knowing