What do smart tabs in WebStorm/PHPStorm do?

If smart tabs option is enabled then only spaces are used for alignment, else tabs and spaces are used.

e.g: for the following code block

    function(first, 
             second) {
    }

without smart tabs

t---function(first, 
t---t---t---.second) {
t---}

with smart tabs

t---function(first, 
t---.........second) {
t---}