A self-assigned "callsign" that the operators give themselves. I.e. a "handle". Some 11m band DX clubs or CB radio groups/clubs assign sequential identifiers as well, for example "World Radio 123" is the 123rd member of the World Radio CB/11m club. A lot of the numbers you'll hear are simply self-assigned numbers that the operator chooses to use as their callsign or identifier for whatever reason.
If/when the band is open internationally again, you'll start to hear number-number-letter-letter-number-number-number type callsigns on 11m (for example, 26AD456, or 2WR123). The numbers before the letters are the 11 meter country prefix (the USA is "2", Italy is "1", England is 26, Canada is 9, Mexico is 10, etc. The two letters are an identifier for the radio club or another identifier the operator uses (so in the case of the World Radio club example, 2WR123 is simply World Radio 123 from the United States, 26AD456 would then be Alpha Delta 456 from England).
International CB and 11 meter operators are more likely to use the NNLLNNN system or prefix system for self-assigned 11m callsigns and a lot of US stations do not use this system, and simply use a three digit number or a handle of their choice, even when talking to stations outside the US.
Here's the whole list if you're interested:
http://27mhz.50megs.com/prefix.html