What does this little hand mean?
It means that they invited you to the topic and want you to participate!
Oh! Thanks! I was really confused about it!
Likewise, if you invite a person (which you can) the same thing will pop up in your invited person's notifications.
How do you invite someone?
Hit the little invite button near the bookmark button!
Go to your profile, click invites, then "send an invite"!
The hand wants you to post in that topic