What does 'source' do?

source is a bash shell built-in command that executes the content of the file passed as argument, in the current shell. It has a synonym in . (period).

Syntax

. filename [arguments]

source filename [arguments]

Be careful! ./ and source are not quite the same.

• ./script runs the script as an executable file, launching a new shell to run it
• source script reads and executes commands from filename in the current shell environment

Note: ./script is not . script, but . script == source script

https://askubuntu.com/questions/182012/is-there-a-difference-between-and-source-in-bash-after-all?lq=1

It is useful to know the 'type' command:

> type source
source is a shell builtin

whenever something is a shell builtin it is time to do man bash.