To add to JeppeSN's correct answer: the second file is a sieve file following the "ABCD" format which is used (for example) by the program OpenPFGW. The full documentation for ABCD (as well as two related formats, ABC and ABC2) is included with OpenPFGW; you can find a download link via google.
The short explanation is that it encodes a very long list of numbers of the form 3*2^a-1, where "a" starts at 25,000,034 and then increments by each of the numbers you see below. (There are gaps because we already have factors for some of these numbers.) The 321 sieve program will determine whether any of these numbers are divisible by a prime between 52,830,560,000,000,000 and 52,830,570,000,000,000; if it finds any, it will report the factors back to the server. We will later get to skip the primality test for 3*2^a-1 for that value of a, since we will already know it's composite.
If you are exceptionally observant, you may notice that the ABCD file actually has another line that looks like the first, a little over halfway down. This is for numbers of the form 3*2^a+1 (instead of -1). We are currently sieving both forms for 25,000,000 <= a < 50,000,000.