A back bet is a wager that a selection will win — the standard, familiar form of betting that has existed for centuries. When you back a football team to win, a horse to win a race, or a player to win a match, you are placing a back bet. If your selection wins, you profit; if it loses, you lose your stake.
On betting exchanges, the term "back" is used explicitly to distinguish it from laying (betting against a selection). On traditional bookmakers, every bet accepted is by definition a back bet — there is no alternative from the bettor's perspective, as the bookmaker always takes the opposing position.
Back bets on exchanges work identically to bookmaker bets in terms of win/loss mechanics. The key difference is in how prices are set. On an exchange, you can either accept the current best available price (the lay side's best offer) or post your own price and wait for a layer to match it. Posting your own price can generate better odds but may go unmatched if the market moves away.
Back-to-lay trading is a common exchange strategy: back a selection at long odds before an event (or before key information emerges) then lay the same selection at shorter odds once the price has drifted or the situation changes, locking in a profit regardless of the outcome. This is green-booking a position.
Example
Before a football match, you back Chelsea to win at 2.80 on Betfair (£50 stake). During the match, Chelsea score first and their win odds shorten to 1.60. You lay Chelsea at 1.60 for a calculated stake to lock in profit. Whatever the result, you are now in profit — this is back-to-lay trading.