The 8-channel relay module contains eight 5V relays and the associated switching and isolating components, which makes interfacing with a microcontroller or sensor easy with minimum components and connections. Each relay on the board has the same circuit, and the input ground is common to all eight channels.
The eight relays on the module are rated for 5V, which means the relay is activated when there is approximately 5V across the coil. The contacts on each relay are specified for 250VAC 30VDC and 10A in each case, as marked on the body of the relays.
The switching transistors act as a buffer between the relay coils that require high currents, and the inputs which don’t draw much current. They amplify the input signal so that they can drive the coils to activate the relays.