CVE stands for Common Vulnerabilities and Exposures. That boils down to it being a database of problems that have been detected, described, and listed for reference for security professionals. It is maintained by the Mitre corporation, which provides one of many interfaces to the data. Mitre also acts as the primary CNA, which means CVE Numbering Authority in this context. There are other numbering authorities, but they act in coordination with each other to assign new numbers to newly identified problems. In this article on Wikipedia, the history and background of the CVE system is discussed.
The video below is from f5, a security business. It is trying to make the need for their services clear in context.
In addition to knowing what a CVE listing is about, it will help to learn something about the process used to determine the severity of entries in the database. The video below is from f5 again, this time explaining the scoring process.
Although the Mitre site is authoritative, it may be difficult to find what you are looking for on it. Another useful site is provided by another security company, Tenable. On it you can search for particular database entries, but you can also look through lists of recent listings and lists of recent updates to see what current dangers have been identified.
In the next video, the presenter puts together several tools you have already seen, using them to hunt for problems on a customer's network.