In a shell, $$ refers to the process id (PID) of the current running process. For example, when used inside a shell script, it would return the process id of the running script. Similarly, when used inside a terminal (e.g. echo $$), it would return the process id of the current shell (as the current running process in that case would be the shell itself). One common use of $$ is to check which shell is being used in terminal.
Using $$ is not considered a good way to generate unique filenames, and should be avoided for such tasks wherever possible because it can lead to vulnerabilities in your shell scripts.
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