Multiplier x price = notional-value. Multiplier x tick size = tick-value. It is the single number that turns a chart into money, and it differs wildly between products that look similar on screen.
Index futures state it directly ($50 x the S&P). Commodity contracts state it as a physical quantity — 1,000 barrels, 100 troy ounces, 5,000 bushels — which amounts to the same thing.
Example: ES has a multiplier of 50, MES of 5, NQ of 20, MNQ of 2. A 10-point index move is $500, $50, $200 and $20 respectively. Same chart, four very different risks.
Related: point-value, tick-size, notional-value, tick-value, micro-futures