Startups in early stages of their development are still looking for a product-market fit and don't have large sales to show to investors. Depending on how far they are with developing their product and finding sales channels, they can receive financing in Pre-Seed or Seed rounds.
The main difference between Seed and Pre-Seed funding stages is that to qualify for the Seed stage the startup team needs to find at least one cost-effective sales channel. If they do so, it is now possible to calculate how much revenue the startup will earn over a certain period of time. At the Seed stage, the startup is already evaluated on the basis of the financial model.
Requirements for a Pre-Seed investment: proven demand, there is an MVP, the team is assembled.
Requirements for a Seed investment: business model is viable, there is a predictable economy of scale. The Seed stage is also considered to be a stage just before the Growth stage of a startup.