A lot represents the number of units of a financial instrument as set out by the exchange.
For example: Nifty future/option contract has a lot size of 50 quantity. It is not possible to buy below 50 quantity because these are exchange standardized contracts and cannot be customized. If you want to buy more, it has to be in the multiples of 50 (Example: 100 - 2 lots, 150 - 3 lots, 200 - 4 lots, 1000 - 20 lots etc.). To know the lot size for each stock, click here.