What is this triangle symbol representing?
They are drivers with enables. Basically they take low-current drive signals (from a microcontroller, for example) and amplify them to high-current capable of driving high-power devices, in this case motors. The wire going into the "back" of the triangle is the input signal, the wire coming out of the "front" of the triangle is the output drive, and the wire going into the "side" of the triangle is an enable input, turning the driver on or off.