What does the 's' in srwxr-xr-x in the 'ls' output mean?

From the ls manual:

The file type is one of the following characters:

‘-’ regular file
‘b’ block special file
‘c’ character special file
‘C’ high performance (“contiguous data”) file
‘d’ directory
‘D’ door (Solaris 2.5 and up)
‘l’ symbolic link
‘M’ off-line (“migrated”) file (Cray DMF)
‘n’ network special file (HP-UX)
‘p’ FIFO (named pipe)
‘P’ port (Solaris 10 and up)
‘s’ socket
‘?’ some other file type

So, that's a Unix socket. It could be meaningful, since sockets are created by processes to listen for requests. Use lsof to determine what process is using that socket.

You may need to use sudo with lsof, if the socket is opened by a process running as another user:

$ lsof /run/snapd.socket
$ sudo lsof /run/snapd.socket
lsof: WARNING: can't stat() fuse.gvfsd-fuse file system /run/user/1000/gvfs
      Output information may be incomplete.
COMMAND   PID USER   FD   TYPE             DEVICE SIZE/OFF   NODE NAME
systemd     1 root  197u  unix 0xffff99dc9afa3000      0t0 191670 /run/snapd.socket type=STREAM
snapd   18626 root    8u  unix 0xffff99dc9afa3000      0t0 191670 /run/snapd.socket type=STREAM

You could use file command to determine its type too:

$ file /tmp/ssh-k405k6mf0/agent.1221
/tmp/ssh-k405k6mf0/agent.1221: socket

or even mimetype:

$ mimetype /tmp/ssh-k405k6mf0/agent.1221
/tmp/ssh-k405k6mf0/agent.1221: inode/socket