Being self-employed means you’re working for yourself. It’s the same thing as being a “sole trader”.
The usual way you’re getting paid as self-employed is by invoicing your clients. You’re also the one responsible for paying Income Tax and National Insurance on this income. You do this by filing a tax return with HMRC.
Examples of self-employed people:
freelancers (writers, interior designers, personal trainers, etc.),
private hire and delivery drivers (Uber, Deliveroo, etc.),
hairdressers renting a chair in a salon,
contractors and consultants who aren’t working as a limited company, and
CIS construction workers.
You are NOT self-employed when:
Your employer pays your salary through PAYE. In this case, they pay your taxes and you shouldn’t need to do anything.
You do business through your own limited company. In this case, even though you work for yourself, HMRC doesn’t consider you to be “self-employed”. You’re actually an employee of your own company, and you can pay yourself through PAYE and/or dividends.
If you’re not sure if you’re employed, ask the company you work for.