Suppose that we need to delete all files with the same extension (i.e., .bak or .txt) from current folder and all its subfolders:

find . -name "*.bak" -type f

This command will find all the files with the .bak extension. Make sure whether these files are safe to delete¹.

Then, append -delete to swipe all these files:

find . -name "*.bak" -type f -delete

Note: make sure the -delete is the last argument in this command. If you put it before the -name, it will delete everything…

See man find and man rm for more information.