A recliner is a chair that reclines. Or to put it another way, the back of the chair can be adjusted to lean back, and there is usually a footrest, so that the person sitting in the chair is near horizontal.
Recliners can be helpful for people with medical conditions that require them to keep their feet up as much as possible or for people who should not sleep in a fully reclined or horizontal position, such as a regular bed. Some recliners have built in table tops or storage pockets, providing a surface for eating, holding medication, reading materials or other conveniences for people who may need to sit in them for extended periods. They are generally built to accommodate large people. Some can also be tilted forward to make it easier for someone to get up. And some recliners have built in massage capabilities.