Lots are how forex position sizes are measured. A standard lot equals 100,000 units of the base currency — so 1 lot of EUR/USD is €100,000 of exposure. A mini lot is 0.1 of a standard lot (10,000 units), a micro lot is 0.01 (1,000 units), and some brokers offer nano lots at 0.001 (100 units).
Lot size determines the dollar value of one pip. On most USD-quoted pairs: 1 standard lot ≈ $10 per pip, 1 mini ≈ $1 per pip, 1 micro ≈ $0.10 per pip. Traders pick lot size based on account size, risk tolerance, and stop distance — a common rule is to risk no more than 1% of the account balance per trade, which directly determines how many lots you should enter.