Meta’s definition of Conversions API is:
Designed to create a connection between an advertiser’s marketing data (such as website events, app events, business messaging events, and offline conversions) from an advertiser’s server, website platform, mobile app, or CRM to Meta systems that optimize ad targeting, decrease cost per result and measure outcomes.
In essence, CAPI is Meta’s first-party data tool for tracking conversions. Instead of relying on third-party cookies through the Meta pixel, CAPI collects first-party data and uses a server to transfer the data over to Meta. From here, Meta can continue to track the conversion server-side, taking away heavy reliance on third-party cookies. However, for CAPI to operate successfully, you need an efficient server to collate and transfer this first-party data to Meta.