The first function is probably familiar from some other interfaces: filter out posts according to certain keywords.
The second is possibly a little less familiar, but the idea would be like following hashtags on other federated microblogging services like Misskey or Mastodon or the like, except instead using keywords since Lemmy currently doesn’t support hashtags.
Emphasis on the “or” here, as an interface that has only one of either is of interest!
I don’t use web lemmy so I’m not sure about the filtering, but I’m relatively confident that none allow keyword following. That’s something that would have to be implemented on the backend