The salesperson who finds and qualifies leads, then hands them to an AE for closing.
SDRs do prospecting: researching accounts, sending cold outreach, making discovery calls, qualifying leads. They don't close deals themselves. They hand qualified opportunities to an AE (Account Executive), who takes over. The split exists because prospecting and closing are very different skills, one is volume + persistence, the other is deal shaping + relationship.
The SDR-AE split is the backbone of B2B sales for deals over $10-20k. If your deals are smaller, self-service might beat SDRs. If larger, you probably want BDRs (outbound SDRs) AND inbound SDRs AND AEs, a small sales org already.