What does m mean in kernel configuration file?

y  =  yes (always installed)
m  =  loadable module (can install and uninstall as you wish)
n  =  no (never installed)

see: https://www.linuxquestions.org/questions/linux-general-1/boot-config%2A-273853/

I assume, this refers to the same as the (y,n,m) prompt when running make config; in that case it would be "module".

Note that compiling Unix domain sockets (CONFIG_UNIX) as module is probably not a good idea. A lot of system components and programs depend on them, and some services might fail to start up if the module has not been loaded at that time.

Most functionality in the Linux kernel can either be compiled in ("y") or left out ("n"), and much of it can also be compiled as a loadable module. This makes sense when you don't know for certain whether you will need some feature in the future.

If you compile it as module and it turns out that it is needed, it will work, but until then it will not bloat the kernel.

It does not, however, really make sense to configure Unix domain sockets as a module, because they are needed almost everywhere (e.g. udev will fail to launch at startup).

If you know you will need something anyway, that should be "y", not "m"