Board Lot in Stock Trading

Each stock may trade at different minimum lot sizes controlled by its price.  This minimum lot size is called a Board Lot. For example, if a stock's board lot is 1,000 then that is the minimum amount of shares that can be bought and any mount above this should also be traded in scales of 1,000. Thus an order to buy a stock with a board lot of 1,000 will reject an order to buy 1,500 and 3,200; but will accept 1,000, 2,000, 5,000 and above as long as it is wholly divisible by 1,000 and not leave any partial lots. 

Any amount of shares not wholly divisible by a minimum board lot is considered an Odd lot. Odd lots are traded at an Odd Lot board which are usually illiquidly traded and often at a discount to the stocks market price.

What Does Board Lot Mean?
A standardized number of shares defined by a stock exchange as a trading unit. In most cases, this means 100 shares. The purpose of a board lot is to avoid "odd lots" and to facilitate easier trading. It's more difficult for a broker to find a buyer for, say, 17 shares, than if everybody agrees to trade in 100 share lots.

Investopedia explains Board Lot
For example, a stock exchange might define one board lot as equaling 1,000 shares for stock priced under $1, and 100 shares for shares of more than $1. The thinking is that standardization increases liquidity thus lowering spreads and making the market more efficient for everybody.

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