Pairing stands for a series of flights, on a single day (up-and-down flight) or on multiple days (including a stopover in outstation). Crew and aircraft may (or may not) change along the pairing.
Here is a sample : Pairing WM24
Monday : Flight 240 from AMS to FRA
Flight 241 from FRA to AMS
Flight 690 from AMS to BCN
Hotel layover in BCN
Tuesday : Flight 702 from BCN to AMS
All 4 flights and hotel stop-over are grouped into a "pairing". Schedulers that create your personal roster do not plan "flights" but "pairings".
You can enable or disable the "Pairing" logging from the Settings - Flight Logging page.