Random refers to when a card or cards are picked from a selection in a way such that the outcome is unpredictable.
The two main ways of doing this are as follows:
1. To hold the cards up, with their backs facing your opponent, and have them choose one at random.
2. Put them into a pile, shuffle them, and then take the desired/required number off the top.
Both of these methods are generally acceptable, although the second can be deemed to be more "purely random".