Emacs mode 1, 2 and t (what t stands for)?

Question

For instance, global-linum-mode 1 enables line numbers global-linum-mode 0 disables line numbers, what global-linum-mode t would do? and I've seen another parameter: nil. What do they do?

Answer 1

t is true, nil is false.

Answer 2

t and nil in boolean context are the (Emacs) Lisp truth values. nil is false. Any non-nil value is considered to be true, but t is customarily used. Source.

As for global-linum-mode, any non-negative parameter will enable it, including t, "foo",... etcetera.

For such questions it is very useful to play in the scratch buffer with elisp: type your expression there and press C-j.