MPTCP is an evolution of TCP that allows a concurrent use of multiple interfaces for a single TCP session. If the both sides of a TCP connection are MPTCP-capable (otherwise a legacy TCP will be used between them), multiple subflows are established over different data paths, i.e., multiple network interfaces, and the MPTCP sender spreads TCP segments over the subflows. Spreading data over subflows is enabled by defining and managing separate congestion windows for individual subflows. TCP’s congestion control mechanism then distributes traffic based on the congestion condition of each data path. When a high congestion condition is detected in a data path, the amount of traffic on it is reduced and other less congested paths convey more. Thus MPTCP accomplishes dynamic load balancing between interfaces.

The detail explanation about MPTCP can be read below.