In short: ESG stands for three groups of issues taken into account in investing alongside profit: environmental (E), social (S) and governance (G).
Simply put: E covers climate, water, waste and nature. S covers working conditions, human rights and equality. G covers how a company is run — honesty, accountability, pay. Different products take ESG into account in different ways and with different rigour. Words like "green", "ethical" or "responsible" in advertising have no common definition.
Example: Two funds use the word "sustainable" in their description. The documents show that one only describes how it takes risks into account (SFDR, Art. 6), while the other has a sustainable investment objective (Art. 9). The label is the same; the promise is different.
Watch out: ESG describes characteristics and objectives; it does not promise a higher return or lower risk.
Source: Regulation (EU) 2019/2088 (SFDR) — http://data.europa.eu/eli/reg/2019/2088/oj; European Commission — https://finance.ec.europa.eu/sustainable-finance/overview-sustainable-finance_en