The expected move is the one-standard-deviation range the options market prices for a stock through a given expiration, readable from the at-the-money straddle or computed from IV scaled by the square root of time. Roughly two-thirds of outcomes are expected to land inside it, one-third outside.
Expected-move units make strike placement comparable across stocks: a strike two dollars away means nothing by itself, while a strike 1.2 expected moves away carries the same implied odds on any stock. Careful placement rules measure distance in expected-move units for the trade's own expiration, computed from the larger of implied and recent realized volatility, so the ruler cannot understate a stock that is actually moving faster than its options price.