Simple Agreement for Future Equity — an investor gives money now in exchange for shares later, when a priced round happens. It is not a loan and has no interest or maturity date.
$250,000 on a SAFE with a $5M valuation cap converts at the seed round into shares priced at no more than a $5M valuation.
SAFEs are fast and cheap, which is why founders stack several of them — then discover at the priced round that they have given away far more than they thought. Model the conversion BEFORE you sign the next one.