In plain English
MCP gives AI applications a shared way to discover and use capabilities exposed by other systems. The AI application is the host; a client inside it communicates with an MCP server that provides tools, data, or reusable prompts. Tool calling is the act of requesting a tool, while MCP defines a common way for applications and servers to communicate about those capabilities.
Example
A startup connects its AI assistant to an MCP server that exposes a tool for searching its product documentation. The assistant can request that tool and use the returned passages to help answer a customer's question.