SSE stands for Server-sent Events.
SSE is used as a mean for a server to send messages to the client over the web. Since HTTP doesn’t offer that capability, a mechanism such as SSE can be used.
The way SSE works is by the web browser opening an HTTP request to the server and the server using that request to endlessly stream its messages to the client without ever finishing the response for the initial request.
SSE is used when the developer expects many messages to be sent from the server to the client.