pgsql database .1 .2 .3 file names

Can anyone tell me what the data file names with the .1 .2 .3 etc.
extensions are? Are these undo versions of my tables or something? I
need to recover some disk space and I'm wondering if I need them. Here
is an example...
-rw------- 1 postgres postgres 1073741824 Oct 18 20:01 19523580
-rw------- 1 postgres postgres 1073741824 Sep 6 20:11 19523580.1
-rw------- 1 postgres postgres 1073741824 Sep 6 20:11 19523580.2
-rw------- 1 postgres postgres 1073741824 Sep 6 20:11 19523580.3
-rw------- 1 postgres postgres 1073741824 Sep 6 20:11 19523580.4
-rw------- 1 postgres postgres 1073741824 Oct 6 20:19 19523580.5
-rw------- 1 postgres postgres 629948416 Oct 19 12:33 19523580.6

Thanks, Bryan