Arbitrage means backing every outcome of an event across different books at prices that, together, cover the whole market for less than it pays out. When the numbers line up, the position carries a margin while those prices hold — it depends on the odds not moving and on staking each side correctly, so it is a market inefficiency, not a payout promise.
In practice the prices move and books can limit accounts, so arbitrage is about spotting and acting on short-lived gaps, not a steady income. Use the Arbitrage and No-Vig Fair Odds calculators to check whether a given pair of prices actually covers the market.