I dont have much technical experience in networking technologies, but I have worked for ISPs, and typically they sold clients a WAN solution, using an MPLS network, which is something like:
Which is a hub-spoke topology, where the branch-offices connect to the head-office, and the head-office has the firewall and break-out to the internet. The obvious down site to this setup is that all traffic must traverse the head-office, which can add latency. With more and more apps now be using from the cloud, all of this traffic needs to go via the head-office first, for no good reason.
There is alot of talk regarding Software Defined Networks (SDN), which breaks up the typical network into a seperate Control and Data plane. Usually, a router carries all the traffic, and the logic to control how traffic is to be sent is also on the same device. So using this new tech, networks can be controlled and programmed, because the control of the network is open and seperate from the layer that carries the traffic.
SD-WAN uses that methodology, and applies it to the WAN elements, which as first mentioned, was based on the hub-and-spoke MPLS design. Using a way to control the way traffic is routed between the hub (head office) and spokes (branches), that allows for traffic to flow directly to the internet and between branches, SD-WAN is something like this:
This avoids latency, and makes special traffic like VoIP much better.