People don't usually want to invest in your wild idea or in the wrong team. If you've had a series A then someone has vetted your idea and your team.

Series A is the first round of investment meaning you've convinced someone other than yourself the product is useful/viable.

Seed funding: There is little to no revenue. There is no business model. The startup is trying lots of different things to see what sticks. Most of the work is building prototypes and throwing them away.

Series A: Startup is making money and has a repeatable and scalable business model. They may still be in the red with fixed costs, but every dollar spent on marketing returns more than $1 in income. The work is usually turning those hacky prototypes into stable code.