It's a read/write protocol for making external data/services available to a LLM. You can write a tool/endpoint to the MCP protocol and plug it into Claude Desktop, for example. Claude Desktop has MCP support built-in and automatically queries your MCP endpoint to discover its functionality, and makes those functions available to Claude by including their descriptions in the prompt. Claude can then instruct Claude Desktop to call those functions as it sees fit. Claude Desktop will call the functions and then include the results in the prompt, allowing Claude to generate with relevant data in context.

Since Claude Desktop has MCP support built-in, you can just plug off the shelf MCP endpoints into it. Like you could plug your Gmail account, and your Discord, and your Reddit into Claude Desktop provided that MCP integrations exist for those services. So you can tell Claude "look up my recent activity on reddit and send a summary email to my friend Bob about it" or whatever, and Claude will accomplish that task using the available MCPs. There's like a proliferation of MCP tools and marketplaces being built.