What is that thing under the chair on Harry Potter seven near end?

You're likely referring to the Horcrux locket that Harry, Ron, and Hermione find in the Shell Cottage. It's hidden under an armchair and is disguised as a simple locket.

Here's why it's significant:

* Horcrux: This locket is one of Voldemort's Horcruxes, a dark magical object that holds a fragment of his soul, making him nearly immortal.

* Hidden: Voldemort left the locket with the intention of having it destroyed, but it ended up in the hands of Regulus Arcturus Black, Sirius Black's younger brother, who later replaced it with a fake to protect the locket from being destroyed.

* Importance: Harry, Ron, and Hermione spend a significant amount of time trying to figure out how to destroy this Horcrux, and their journey is filled with challenges and personal struggles.

Let me know if you have any other questions about the Harry Potter series!