You've gotten good answers from other folks but I'll provide a ELI5:

Basically a set of rules in the database to make sure that it is immediately consistent.

NoSQL databases offer eventual consistency in exchange for speed so they are generally not considered to be ACID compliant.

Most traditional databases (MySQL, postgresql, etc.) are.

There are a couple of emerging companies that try to tackle speed for traditional databases. CockroachDB offers a postgress-based database that scales more like NoSQL while still offering ACID transactions.

TiDB is a similar company but for MySQL.