Every year, money enters the government's coffers. This is its annual revenue.

歳入 specifically refers to annual government revenue, rather than the revenue brought in by a company or an individual. It's the money that a national or local government collects in a single fiscal year, also known as fiscal-year revenue.

This is a jukugo word that uses the on'yomi readings of the kanji. You should be able to read this on your own.