In order for your buy order to execute, another user on the platform must place a sell order that matches the price you placed your buy order at.
In order for your buy order to fully execute, the amount you are trying to buy must match the amount users are willing to sell. If the amount you are trying to buy exceeds the amount being sold, your order will only partially execute.
The same goes for sell orders. For your sell order to execute, another user on the platform must place a buy order that matches the price you placed your sell order at.
In order for your sell order to fully execute, the amount you are trying to sell must match the amount users are willing to buy. If the amount you are trying to sell exceeds the amount being bought, your order will only partially execute.