Wort is the sugary liquid pulled from mashed grain, before yeast turns it into beer. Every step before pitching — mashing, lautering, sparging, boiling — exists to produce wort with the right sugar concentration, the right balance of fermentable and unfermentable extract, and the right level of hop bitterness dissolved into it.
Wort gravity is what a hydrometer actually measures
When a recipe calls for hitting a target original gravity, it's wort gravity being measured — the dissolved-sugar density of this specific liquid, read before fermentation has consumed any of it. That reading is also what feeds directly into the IBU calculator's boil gravity input, since hop utilization depends on how concentrated the sugar in the boiling wort is, not on the batch's eventual finished strength.
Pre-boil versus post-boil wort aren't the same liquid
A batch's pre-boil wort and its post-boil wort differ meaningfully in volume and concentration — evaporation during the boil concentrates the sugar further, which is exactly why a boil-off-rate calculation matters for hitting a target final gravity rather than just measuring gravity once and assuming it holds steady through the whole brew day. A brewer who only checks gravity after the boil has no way to catch a mash or lautering problem until it's already too late to fix that batch.