MITM means Man-In-The-Middle. MITM proxies are usually deployed by companies in "enterprise environments" and elsewhere, where the owners of the network have a desire to investigate even TLS encrypted traffic.
To do this, they require users to install a custom "trust root" (Certificate Authority (CA) certificate) in the client, and then the proxy terminates all TLS traffic from the client, impersonates the remote server and acts like a proxy. The proxy then sends back a generated certificate signed by the custom CA. Such proxy setups usually transparently capture all traffic from clients to TCP port 443 on a remote machine. Running curl in such a network would also get its HTTPS traffic captured.
This practice, of course, allows the middle man to decrypt and snoop on all TLS traffic.