DI (digital input) - controller input has two states: either the voltage is applied or not applied.
DI is used to connect reed switches, water and electricity meters, buttons. To do this, one contact of the button/reed switch/meter is connected to a voltage source (for example, to the V out terminal), and the other - to DI. The DI state(zero or one) indicates whether the circuit is closed or open.
The voltage threshold for guaranteed operation (transition to the voltage supplied state) is 3.3V. the maximum possible voltage can be found here: Wiren Board 4:Hardware#Terminal blocks.