In short: an investment scam is an offer to put money into a "safe" opportunity with high returns, with no licensed provider and no real investment behind it — the money is simply taken.
Simply put: It often starts with an advert or a message, followed by a "personal adviser". The platform shows growing "profits", but you can't withdraw — or you must first pay a "fee" or "tax". The red flags: guaranteed high returns with no risk, pressure to decide quickly, an unlicensed seller, an unclear product.
Example: You pay in €250; a month later the screen shows €900. When you ask to withdraw, you are told to pay a €300 "release fee". That is a second scam on top of the first.
Watch out: numbers on a screen are just numbers. The real test is whether you can withdraw the money to your bank.
What to do: five questions before any investment — who is the licensed provider, where does the return come from, can I get out, what is on paper, can I lose everything. Check the financial supervisor's register.
Source: ESMA — warnings and publications for investors: https://www.esma.europa.eu/investor-corner/warning-and-publications-investors · Example: Bulgaria — Financial Supervision Commission (FSC), registers and warnings: https://www.fsc.bg