Ask - It allows people to ask or send a message through your tumlr ask box.
Reply - It allows followers to directly reply to your posts in their dashboards.
Submit -It allows people to submit pictures,videos,music and etc.
If you are wondering why you feel alone while on tumblr then you probably forgot to enable these=))
1.Go to Customize
2.Find The COMMUNITY on the menu beside the SAVE and CLOSE buttons.
3.Check everything.
4.Save and close=)