"tramps who come wearily down from the highway in the evenings to jungle-up near the water"

Of Mice and Men is set during the Great Depression, a time when jobs were scarce and many men took to the roads in search of work. These men were called "tramps" because they tramped (walked) from town to town looking for work.

Tramps often gathered together at night for companionship and safety; these gatherings were called "jungles."