What does `. /path/command` do? (Space after dot, absolute path)

From this guide to Bash completion we learn that in order for Bash to perform auto-completion one must perform . /etc/etc/bash_completion.d/foobar (note the space after . ) in order for Bash completion to work.

$ /etc/bash_completion.d/ssh
bash: /etc/bash_completion.d/ssh: Permission denied
$ . /etc/bash_completion.d/ssh
$ ls -l /etc/bash_completion.d | grep ssh
-rw-r--r-- 1 root root   297 Jan 28 18:04 ssh

Is . a shortcut for the source command? If not, then what is it? It is impossible to google for, man source returns nothing, and apropos source and info source give so much irrelevant information that I cannot tell if what I'm looking for is in there. How might I even begin to RTFM to find the answer to this question myself?