The \ on it's own is used to escape special characters, such as \n (new line), \t (tabulation), \" (quotes) when typing these specific values in a System.out.println() statement.
Thus, if you want to print a backslash, \, you can't have it on it's own since the compiler will be expecting a special character (such as the ones above). Thus, to print a backslash you need to escape it, since itself is also one of these special characters, thus, \\ yields \.