. is a command.
It's a shell built-in command, that reads the named file and executes the commands therein in the current shell process.
The Bourne Again shell also has
source as a synonym for this command. But this is a Bashism (that the Bourne Again shell took from the C Shell). Albeit it is a Bashism that the Bourne Again shell shares with the TENEX C Shell, the Z Shell, and others (but not the Korn shell, note). The Single UNIX Specification only standardizes
Also note that the behaviour of
source subtly changes dependent from whether the Bourne Again shell is being run in its POSIX-compatible mode or not. (Again this is like other shells, although their non-standard behaviours are not the same as one another's. With the Z Shell, for example, there is a precompiled shell script mechanism, and
source subtly differs from
. in its search path handling. The Korn shell's
. will run shell functions, for another example.)
~/.bashrc is merely one of several files whose contents are (dependent from how the shell process is invoked) automatically sourced at shell startup. There's nothing prohibiting it from being manually sourced. Although if its actions aren't idempotent, you might have some fixup work to do afterwards.