These variants of Mouse-1 select entire words or lines at a time. Emacs activates the region around the selected text, which is also copied to the kill ring.
Double-clicking on a character with “symbol” syntax (such as
underscore, in C mode) selects the symbol surrounding that character.
Double-clicking on a character with open- or close-parenthesis syntax
selects the parenthetical grouping which that character starts or
ends. Double-clicking on a character with string-delimiter syntax
(such as a single-quote or double-quote in C) selects the string
constant (Emacs uses heuristics to figure out whether that character
is the beginning or the end of it).