In short: an investment fund pools the money of many people, and a manager invests it in many assets; each person holds units whose price changes with the value of the assets.
Simply put: Instead of buying dozens of securities yourself, you buy a share of a common "pot". In the EU, funds open to a wide range of investors follow the UCITS framework (Directive 2009/65/EC): authorisation, a management company, a depositary, rules on investing and on disclosure. The framework does not remove risk.
Example: You invest €1,000 in a fund whose unit is worth €10 and receive 100 units. If the unit value becomes €9, your holding is €900; if it becomes €11, it is €1,100.
Watch out: "a fund means safe" is a myth. Risk can range from very low to very high — read the key information document and the fees.
Example: Bulgaria — funds and their management companies are supervised by the Financial Supervision Commission (FSC) — https://www.fsc.bg
Check in your country — your national financial supervisor and its public register of authorised funds.
Source: Directive 2009/65/EC (UCITS) — https://eur-lex.europa.eu/eli/dir/2009/65/oj