No-deposit bonuses are a customer acquisition tool. The most common version is a $30 credit from XM on new registration, or Exness' occasional promotional credits. You can trade the bonus like real money, but any profit is usually subject to a volume-trading requirement (e.g. trade 0.5 lots total before the profit is withdrawable).
These promotions are legitimate, but the withdrawal conditions are often tight enough that most recipients never meet them. They are best thought of as a free risk-managed trial of the broker's platform rather than a genuine windfall. Check the bonus terms before building a strategy around it.