diodes exercise logic gates?
It's digital logic, 3 being High and 1 being Low, anything in between won't have much effect.

The left side is an OR gate. So its kinda like High or low = 1 or 0 = High
High being 3 volts, and low being 2 or 1 volt relatively speaking. 

In the latter, right side, is like an AND gate, So it's kinda like High and Low = 1 and 0 = 0
Again, High being 3 volts and Low being 1 volt.