In software development, rolling release is a model where updates to a software are continuously rolled out rather than in batches of versions. This way the software always remains up-to-date. A rolling release distribution follows the same model and it provides the latest Linux kernel and the software version as they are released.
Arch Linux is the most popular example of a rolling release distribution however Gentoo is the oldest rolling release distribution still in development.
A plain simple English explanation with some FAQ at Linux Jargon Buster: What is a Rolling Release Distribution? – It’s FOSS
What is rolling release? What is a rolling release distribution? How is it different from the point release distributions? All your questions answered.