The HTTP Upgrade header is a mechanism in the HTTP protocol that allows a client to request the server to switch to a different communication protocol, such as WebSockets, within the same connection. This is commonly used to enhance communication capabilities, for example, upgrading from HTTP to WebSocket for real-time, bidirectional data exchange. The client sends an 'Upgrade' header in its HTTP request, and if the server supports the requested protocol, it responds with an agreement to switch. This process is crucial for enabling modern web applications that require persistent connections and efficient data transfer.