If you can tell what the author is trying to say, please edit the post to fix it if you can. (Leaving a comment saying you did so and asking for confirmation, particularly if the poster is new to the site, is helpful.)
If you can't tell what the author is trying to say and no one else has already done so, you can leave a comment asking for clarification. If this results in a discussion in comments and then an edited post, please go back and flag the now-obsolete comments so they can be cleaned up.
If you don't know what to do and the post just showed up, I usually wait to see if someone else who's more perecptive than I am figures it out. If some time has passed and it's still incomprehensible, though, I recommend voting to close ("can't tell what's being asked") if it's a question and you have the rep, or otherwise flagging for moderator attention.
Thank you to everybody who helps improve Mi Yodeya in this way.