Calculation: roughly the at-the-money straddle price for the relevant expiration. For a $100 stock with a $5 ATM straddle, the expected move through expiration is approximately ±$5. More precise versions use a multi-strike weighting, but the straddle approximation is accurate within a few percent for liquid names.
Use cases: sizing iron condors (place wings outside the expected move for higher probability), evaluating earnings trades (compare your directional thesis vs the implied move — if you expect a smaller move, sell premium; larger, buy premium), and gut-checking strike selection (an OTM strike inside the expected move has a coin-flip outcome, whereas one outside the move is genuinely OTM-probability).