Delta is the option's price change per $1 move in the stock: calls run 0 to +1, puts 0 to -1. It triples as the position's stock-equivalent exposure, an approximate probability of finishing in the money, and the hedge ratio market makers trade against.
A 0.30-delta call gains about $0.30 on the next $1 rally and behaves like 30 shares. Read as odds, the market prices roughly a 30% chance it finishes in the money, which is why credit-spread disciplines place short strikes by delta band. Delta is not constant: gamma moves it as the stock moves, and charm drifts it as time passes.