Outs, odds and the price of a call

When you're behind, count your outs, the cards that turn your hand into the winner. A flush draw has 9, an open-ended straight draw 8, two overcards about 6. Then use the rule of 2 and 4: outs × 2 ≈ your % chance to improve on the next card; outs × 4 ≈ your chance by the river when both cards are coming.

Compare that to the price. If the pot is 10 BB and the bet is 5 BB, you pay 5 to win 20, so you need 25% equity (bet ÷ (pot + 2 × bet)). A flush draw at ~36% by the river takes that price. A gutshot at ~16% folds. That comparison, your chance against the price, is the core calculation of the game, and the Odds trainer drills it until it's quick.