A lot is a standardised unit of trade size that sets how many units of an instrument you buy or sell in one transaction. It is the building block for position value, pip value, and margin.
In forex, a standard lot is 100,000 units of the base currency. A mini lot is 10,000 units, a micro lot is 1,000 units, and a nano lot is 100 units.
A lot sets the size of the position, not the borrowing behind it, which is what separates it from leverage. A larger lot raises exposure and the money value of each pip, while a smaller lot cuts exposure and helps you keep risk in check.
You open 1 standard lot of EUR/USD.
Because 1 standard lot is 100,000 units, you control 100,000 euros against the US dollar.
If you open 0.01 lot instead, that is 1 micro lot, or 1,000 units of the base currency.