The first traffic source that most start-ups use is paid traffic. Paid traffic gives you time to allow your other traffic channels to grow and evolve. It allows you to get product market fit, find those those first customers and find out if you have product market fit.
The most common paid traffic sources are Google Adwords (PPC), Facebook ads, display ads, retargeting, paid influencers and paid content promotion.
With paid traffic you are able to contribute content and engage with the audience. You might be able to control the conversation, but you do not own anything else (data, relationship, creativity, etc. etc.). Hence the name rented traffic.
Without the initial steady stream of visitors from paid traffic, you might never get enough data to growth hack your way to success. That is why paid traffic is the most important traffic source for many startups. Over time, that might change, but it is rare to see a startup succeed who does not buy their initial visitors.