Original Gravity (OG) is the specific gravity of the wort measured before fermentation begins — a direct measure of how much sugar is dissolved in it, and the reference point every attenuation and ABV calculation starts from.
The number that pairs with FG to answer almost every fermentation question
OG on its own tells you how much fermentable material a batch started with; paired with final gravity, it answers the two questions homebrewers ask most — how strong did this turn out (via the ABV calculator) and how much of the available sugar did the yeast actually consume (apparent attenuation). Neither figure means much without the other.
When and how you take the reading actually matters
OG should be measured from well-mixed wort, ideally after the boil and any hot- or cold-break material has settled or been accounted for, and at or corrected to your hydrometer's calibration temperature — see gravity temperature correction for why a reading taken straight off a hot kettle without correcting reads artificially low and throws off every downstream calculation that depends on it. A hydrometer read at wort-boiling temperature and one read at room temperature can disagree by several points on the same actual liquid, enough to visibly shift a recipe's projected ABV.