What does all-in pre-flop mean?
A player bets all their chips before the flop is dealt. If someone calls, there is no more betting: the five board cards are dealt and the best hand wins.
What are the odds of AA against KK pre-flop?
Pocket aces win about 82% of the time against pocket kings. Kings win about 18%.
What is a coin flip in poker?
An all-in between a pair and two higher cards, such as QQ against AK. It's close to 50/50: QQ wins about 57% against AK offsuit.
What is the best hand to go all-in with before the flop?
Pocket aces. Against any single other hand they win at least about 77% of the time. Their toughest opponents are suited connectors like 7♥6♥.
How often does the underdog win?
As often as its chance says. A hand with a 20% chance gets there about one time in five, which is why bad beats feel so common.