In short: A labour code is the main national law on relations between workers and employers: the employment contract, working time, rest, leave, pay and dismissal.
Simply put: It sets the rights a contract cannot take away: a written contract, normal working hours, minimum paid leave, regular pay, equal pay for women and men. It also says when and how young people may work. In some countries these rules sit in several laws rather than one code.
Example: An employer says "we give 15 days' leave". Where the law sets a minimum of 20 working days, such a contract breaks the law.
The EU rule: EU directives set minimums that national law must respect — for example, a maximum average 48-hour week and at least four weeks' paid leave (Directive 2003/88/EC, Articles 6–7).
Example: Bulgaria — the Labour Code: a normal week of up to 40 hours and day of up to 8 hours (Article 136); at least 20 working days' basic paid leave (Article 155(4)); monthly pay (Article 270); equal pay (Article 243); work from 16 with a labour inspectorate permit (Articles 301–303).
Check in your country — Which act holds these rules; always read the version in force.
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