An arithmetic progression is a sequence of numbers with a common difference between any two successive numbers in the sequence. For instance, the sequence 3, 5, 7, 9, 11, 13, 15, ... is an arithmetic progression with a common difference of 2.
See https://www.primegrid.com/forum_thread.php?id=7022&nowrap=true#98979 for more details.
When you say you found a AP, you in fact found at least 20 primes. These primes are way smaller than the other primes found here.
Prime: a single prime, with some specific properties. Look at the description (click on the project name in the home page) for more details on each sub-project.