What is meant by partially filled order?

 Partially filled order means that only a portion of the units in an order have been executed. This can happen if the availability of the units in the market (if you make a buy order) or the availability of market players who want to buy a number of units at the price you offer (if you make a sell order) is not fully available.

Here is an example of how your order details will look if it is only partially fulfilled: