source is a Unix command that evaluates the file following the command, as a list of commands, executed in the current context.
Frequently the "current context" is a terminal window into which the user is typing commands during an interactive session. The source command can be abbreviated as just a dot (.) in Bash and similar POSIX-ish shells. However, this is not acceptable in C shell, where the command first appeared.
Some bash scripts should be run using the source your-script syntax rather than run as an executable command, e.g., if they contain a change directory (cd) command and the user intends that they be left in that directory after the script is complete, or the script file does not have the "execute" permission. Passing the script filename to the desired shell will run the script in a subshell, not the current context.