A feed reader is a software that allows you to read feeds. The main supported formats are rss and atom. They are a great way to keep up with sites from all over internet.
There are two types of feed readers: desktop and online ones. Desktop feed readers (also known as offline) are programs that locally, on your machine. They often download (cache) the content from articles on your disk, letting you read even without an active connection. Online ones live on a remote server.
Miniflux
FreshRSS
Tiny Tiny RSS