Lifespan

A lifespan is the amount of time that an individual will live for. After the Lifespan expires they will die.

Appearance
With Shinigami Eyes, the remaining lifespan of a human is visible by being seen written in floating text above a human's head, as well as the human's name.

Shinigami do not have the power to view the lifespans of other Shinigami. Likewise, humans with Shinigami eyes cannot see the life span of Shinigami and also the owners of Death Notes

Purpose
Shinigami use Death Notes to kill humans and then they will inherit the remaining lifespan from the human they killed with a Death Note.