Could you please explain to me what "at-will" means regarding employment. Also. is Maine and AT-Will state? How would I find out?

Every state except Montana (including Maine) is an at-will state, and even Montana follows the at-will doctrine in some circumstances.

Simply, in an at-will state unless you have a bona fide contract that says otherwise, you can quit at any time and for any reason, and your employer can fire you at any time and for any reason that does not violate the law.