In short: Paid annual leave is the number of days a year an employee may rest while still being paid; the law sets a minimum, and a contract may give more.
Simply put: Leave is part of a job's full package — when comparing offers, look at the days off too. Freelancers and contractors have no paid leave by law, so those days must be built into their price.
Example: Two offers: €1,600 gross with 20 days' leave, or €1,550 gross with 25 days. Five extra days of rest also have a value.
The EU rule: every worker is entitled to at least four weeks of paid annual leave (Directive 2003/88/EC, Article 7(1)); national law may give more.
Example: Bulgaria — basic paid annual leave is "not less than 20 working days" (Labour Code, Article 155(4)); for workers under 18, at least 26 working days (Article 305(4)). The amount of leave must be stated in the contract (Article 66(1)).
Check in your country — The statutory minimum, and what happens to leave you have not used.
Source: Directive 2003/88/EC: https://eur-lex.europa.eu/eli/dir/2003/88/oj · Labour Code (Bulgaria): https://www.lex.bg/laws/ldoc/1594373121