Pre-qualification is a quick, informal estimate of how much mortgage you might qualify for. It's based on a brief conversation about your income, debts, and credit — usually no documentation is verified.
Pre-qualification is useful at the very start of house shopping, when you just want to know what's realistic. It's not strong enough to make an offer with — sellers and agents look for pre-approval letters instead.
Don't confuse the two: pre-qualification ≠ pre-approval. Move from pre-qual to pre-approval as soon as you're serious about shopping. The pre-approval requires real documents (pay stubs, W-2s, bank statements) and pulls your credit.