why does SE Hinton named the book “the outsiders”？what does the word“outsider” signify？ who are the outsiders in the story？

The title refers to people who do not fit into the mainstream of society. In this case the Greasers are sen as lower class and not a part of "respectable" society, they are "outsiders".

Yes

Everyone is an outsider in this book especially the bruins

the outsiders means a not belonging to a particular grouup