In short: a share is a piece of ownership of a company — it gives a right to part of the profit (through dividends), a vote and information, and its price can rise or fall.
Simply put: Behind a share there is a real company — sales, property, profit. As a shareholder you can vote at the general meeting and receive information. A share can be sold quickly, but at an unknown price — so it is not "almost cash".
Example: A company is divided into 1,000,000 shares. If you hold 1,000 of them, you own 0.1% of it. If the company distributes €500,000 in dividends, your part is €500 before tax.
Watch out: buying a share only "because everyone is buying it" is speculation, not investing. The test: what do you get apart from the hope that the price will rise?
Source: EU/OECD-INFE (2022), competences 215–217 (shareholder rights) — https://finance.ec.europa.eu/publications/commission-and-oecd-infe-publish-joint-framework-adults-improve-individuals-financial-skills_en