Block trades are large trades with at least 10,000 shares of stock. Because of their large size, block trades can normally only be initiated by institutionals.
Block trades volume help trades measure the activity of institutional investors for a particular stock or security.
Depending on the liquidity of a particular stock, block trades or block orders can affect the market price of that stock; unless these block trades occur directly between two institutions. In that case, the institutions can agree on a fixed price and the transaction will occur at that price.
In bonds trading, a trade is usually considered a block trade when the transaction exceeds $200,000.