What is the significance of the last line in the novel Of Mice and Men?

The line at the end of the novel can be interpreted to mean that Slim and George are close friends now.

George and Lennie have always been friends, but they could never be equals.  George was Lennie’s protector.  When George met Slim, he was impressed with the fact that George and Lennie went around together.  Most men on the ranch were loners, always going from one place to another by themselves and never staying one place for long.

Slim seemed almost jealous for what George and Lennie had.  He was an impressive guy who had a good reputation on the ranch.  He interrogated George and Lennie from the beginning, curious about how they could travel in a pair and look out for each other.

"You guys travel around together?" His tone was friendly. It invited confidence without demanding it.

"Sure," said George. "We kinda look after each other." (Ch. 2)

What George and Lennie have is unique because Lennie is completely dependent on George.  George looks out for him, protects him, and defends him.  He also gets him out of trouble.  When Lennnie gets into trouble for killing Curley’s wife, George has more trouble than he can handle.  He has come to trust Slim, and Slim is the one who tries to comfort him.

Slim came directly to George and sat down beside him, sat very close to him. "Never you mind," said Slim. "A guy got to sometimes." (Ch. 6)

Slim understands the relationship between Lennie and George better than anyone else.  The other men assume that Lennie was dangerous and just tried to kill George, but Slim knows better.  He knows Lennie would never attack George, but that George had to do what he had to do to protect Lennie.  He tells George they will get a drink to show him that he is on his side.

The last line of the book indicates that Slim and George understand each other, but the other men on the ranch do not.  This puts Slim and George in a special kind of relationship, the friendship that both of them need.  They have the potential to go around together now like George and Lennie did, or George can stay put for a bit.