Also called "channel bonding," this is an advanced technique that lets a router combine multiple internet connections together into a virtual single connection for faster performance. So, for example, a router that supports bonding could combine two 4G hotspot connections into a faster, single connection.
This feature, when it is supported in router hardware, is generally found on high-end gear, and usually requires a dedicated server to combine the traffic together (often resulting in an expensive subscription service fee). The server and subscription costs plus the technical complexity tend to relegate this option to users with extreme geek skills or highly specialized needs.
Bonding can also be accomplished with software and a VPN on a per-device basis. For example, one could bond multiple USB tethered devices (and even Wi-Fi) together on a PC with the very affordable Speedify software and service. Speedify also works with smartphones, combining cellular and Wi-Fi connections.
A Windows PC can also become a virtual router, allowing you to share the bonded connection using the Connectify soft-router.