The number is an injection in summer and a withdrawal in winter, compared against the prior year and the five-year average. Because storage is the only buffer in a market that cannot easily import or export on short notice, the level relative to normal is effectively the gas market's inventory-to-demand ratio.
Gas reacts harder to this number than oil does to its equivalent, because gas demand is weather-driven and supply is inelastic week to week. Moves of 5% on a surprise are unremarkable.
Example: consensus expects a 78 Bcf withdrawal and the report shows 112 Bcf, leaving storage 8% below the five-year average. Front-month gas rallies from $3.20 to $3.42, worth $2,200 per contract.
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