A pattern is a musical phrase of one or more measures that you can store off to one side. It saves you from having to enter the same phrase over and over. Using a repeat sign saves ink, paper, and is easier to understand than seeing the same combination of notes over and over.
Patterns are the sequencer equivalent of a repeat sign.
You start with one or more blank measures, fill them with notes, then assign the group a unique Pattern number from 0 to 99. Then whenever you want to use that Pattern, you plug it into your song by referring to its pattern number. Perhaps the most common use of Patterns is for drum sequences. You would spend a bit of time layering a snare, kick drum, hi-hat, ride cymbal, and whatever else tends to repeat for several bars. Then you would insert this Pattern into your drum track as needed.