In short: return is how much an investment has gained (or lost) over a period, expressed as a percentage of the amount invested — from the change in price plus any income received.
Simply put: The formula is (new value − old value) / old value × 100. The nominal return is the one written down; the real return is what remains after inflation. A higher expected return usually comes with higher risk. Past returns do not promise future ones.
Example: You buy a unit for €1,000. A year later it is worth €1,050 and you have received €20 of income. The return is (1,050 + 20 − 1,000) / 1,000 = 7%. If inflation is 3%, the real return is about (1.07 / 1.03) − 1 ≈ 3.9%.
Watch out: losses and gains are not symmetrical. A 20% fall needs a 25% rise to get back to the start; a 50% fall needs a 100% rise.
Source: EU/OECD-INFE (2022), competences 182, 186 — https://finance.ec.europa.eu/publications/commission-and-oecd-infe-publish-joint-framework-adults-improve-individuals-financial-skills_en