The official name of the BBC programme commonly shortened to Dragon's Den is Dragons' Den.
The British title uses the plural possessive “Dragons'”, because the den belongs figuratively to several investors. In ordinary speech and search queries, people often write “Dragon's Den” with a singular possessive, but the programme’s official styling is “Dragons' Den”.
The series features entrepreneurs pitching businesses to a panel of investors. A successful deal normally involves the entrepreneur offering equity in return for capital, although an on-screen agreement can still be subject to later due diligence and may not always complete.
The apostrophe is a small but distinctive detail in the title. It also explains why the British programme should not be confused with similarly named fictional stories or with international versions that use different names. The BBC title is the canonical form for the UK series.