What it is
An API (application programming interface) exposes functions or data of a service so another one can use them. It is how your CRM, your website and your AI tools connect.
Why it matters to an agency
Almost every integration goes through APIs: connecting GoHighLevel with a tool, sending data to a model, or bringing results into your app.
API vs MCP
An API is called by a developer writing code. An MCP wraps tools so the agent can use them through natural language. The MCP usually relies on APIs underneath.