What is diode equation?

It is an equation that describes the I-V characteristic of a diode. In other Words, how the current depends on the voltage.The Shockley diode equation uses an exponential expression. See the Wikipedia article on "Shockley diode equation" for more details.

However, for many practical purposes, it's accurate enough to think of a diode as being "open" in the "forward" direction (no voltage drop), and having a voltage drop of about 0.7 V (in the case of diodes made from silicon) in the "reverse" direction.