It probably means they've chosen a visibility mode that doesn't allow boosting. More info on post visibilities at https://fedi.tips/who-can-see-my-posts-in-mastodon-how-do-i-send-dms-in-mastodon/
Unfortunately the official apps don't show a post's visibility very clearly, you might want to try a third party app instead? (For example Tusky on Android or Ice Cubes on iPhone)