Multi Protocol Label Switching is a switching technology used mostly by service providers. It allows them to build private WANs that are agnostic to the network protocol and type of addressing used. So for example, multiple customers can use the same private IPv4 address space without interfering with each other. Other customers can have a layer 2 WAN, all on the same provider network.
When a carrier provisions a private WAN for a customer, it's called a MPLS VPN. You can consider it a type of tunneling protocol, because it tunnels the customer traffic over the carrier network.
IPSec VPN is another tunneling protocol for IP only, and only point to point. IPsec offers encryption and authentication, while MPLS VPN doesn't.
isn't WAN connections impossible to be eavesdropped due to the amount of multiplexing and different traffic running through it?
It's not difficult for the carrier (how else would they route your traffic to the right place?). And it's not that difficult for governments to "ask" carriers to do it.