In an operating company, founder shares are simply early common bought at near zero cost. In a spac they are a defined feature: the sponsor buys roughly 20% of the post-IPO equity for about $25,000, known as the promote, and those shares convert to ordinary stock at the de-spac.
The promote is why SPAC economics are hostile to late buyers. The sponsor's stake is nearly free, so the sponsor profits on almost any completed deal while public holders need the stock to hold value.
Example: a $200M SPAC sells 20M units at $10. The sponsor holds 5M founder shares bought for $25,000. At a $10 deal price the sponsor stake is worth $50M and public holders have been diluted by 20% before the target's business is even considered.