Contact resistance is the electrical resistance across a mated contact pair, where the pin meets the socket. It is normally very small, usually in the milliohm range, and the datasheet gives a maximum value for the contact as supplied.
It matters most in power circuits. The heat produced in a contact is current squared times its resistance, so a few extra milliohms at high current can add noticeable heat and voltage drop. Every mated pair in the path adds its own share, so the total grows with the number of connectors in a harness.
Contact resistance is not fixed for life. Wear, contamination, fretting from vibration, and plating damage can raise it over time, which is why datasheets quote a limit after environmental or life testing as well. For a power design, include the worst-case value in your voltage-drop and temperature-rise budget instead of the typical one.
