Examples:
- ":r !date" or ":r !echo hi" to insert a date (or other command) into the open file at the cursor
- ":w !python" or ":w !bash" to run the current buffer through python/bash (or any other command)
- ":%!jq" or ":%!sort" to take the entire file's contents and pipe it into an external command like jq or sort

etc. I find it really useful at times but it seems like few people use it, let alone the commands!

#vim