A demon seal is used to seal a demon.
It was mainly used by the Orochimaru clan to put demons into test subjects' bodies to see how they'd react. Most died due to the power, but some did survive.
Ricileon Orochimaru has one on her stomach so she can control her dark side better. Hell's Amulet has one on the back of the gem to keep what's in it inside.
The three demons in Hell's Amulet have seals on their bodies when they turn human. Sumbato has one on his forehead, Dekku's is on his foot, and Mujika's is on his chest. If the seals are damaged they'll turn back to normal.
The seal's one and only weakness is that (depending on what's inside) it will fade after six years. It'll fade in 2 weeks if a demon is in his human form.