grep 'foo*'

when wanting to match food and foosball, but not mofo or keyfob.

grep '^foo' 

As a glob, foo* means "Any string starting with foo", e.g. food and foosball.

As a regular expression, "foo*" means "f followed by 1 or more o's, anywhere", e.g. "mofo" or "keyfob".

This construct is way more common as a glob than as a regex, so ShellCheck notifies you about it.

If you're aware of the above, you can ignore this message. If you'd like shellcheck to be quiet, use a directive or 'fo[o]*'.