So you mean the standard text for BuddyPress status updates in the activity stream?
These are in the BuddyPress template files provided with the theme. You could edit them there and/or add other explanatory text.
e.g. in activity\post-form.php
This is the form that appears in the main activity directory page.
Of course you will need to watch out for any changes when you update the theme, or I’m not sure whether a child theme’s replacement BuddyPress templates are picked up. If they were that would be the best place to put them.
The other way to edit the text in these files is to use a language file.