Every call in poker has a price — the minimum percentage of the time you need to win to break even, known as your pot odds. Equity is what you bring to that comparison: your actual probability of winning the pot from this moment, averaged over every card still to come. Get a feel for this one number and most calls, bets, and folds start to answer themselves.

What equity actually is

Equity is your probability of winning the pot, with ties split. If you would win six times out of ten and chop once, your equity is 65% (the six wins plus half of the chop). It is always measured from this moment, averaging over every card still to come.

Two flavours show up at the table:

Hand versus hand. You know (or assume) the exact two cards your opponent holds and ask how often you beat them. This is the clean, countable version, and the one the equity puzzle drills.
Hand versus range. You do not know their cards, so you weigh your equity against the whole set of hands they would play this way. That is hand reading, a lesson of its own; master the hand-versus-hand maths first.

Counting it at the table: outs and the rule of 2 and 4

You will not run a simulation mid-hand. Instead you count outs (the cards left in the deck that turn your hand into the winner) and convert them into a rough percentage.

On the flop, with two cards to come: equity ≈ outs × 4.

On the turn, with one card to come: equity ≈ outs × 2.

That is the whole shortcut — the rule of 2 and 4. A flush draw has nine outs (thirteen cards of the suit minus the four you can see), so on the flop it is about 9 × 4 = 36% to get there by the river. On the turn, with one card to come, the rule of 2 gives 9 × 2 = 18% (the exact figure is 9 ÷ 46 ≈ 19.6%, but 18% is close enough to guide a real decision).

A few worth committing to memory, because they recur constantly:

Flush draw: 9 outs → ~36% on the flop, ~19% on the turn.
Open-ended straight draw: 8 outs → ~32% / ~17%.
Gutshot: 4 outs → ~16% / ~9%.
Two overcards: 6 outs → ~24% / ~13%.

And four preflop reference points, heads-up:

A pair against two overcards is close to a coin flip, usually 52–57% for the pair (the classic "race").
One card dominated (A-K versus A-Q): about 70% for the better kicker.
Pair over pair (Q-Q versus J-J): about 80% for the higher pair.
Two overcards versus two undercards (A-K versus Q-J): around 60%.