What is truncated index in Wagner Algorithm (Equihash)?

Hello, I code my Equihash implemenation. It is working, but i don’t understand that optimization like truncating indices. What is it?
That is a index in 3 bytes, because index’s value always < 2*21? I tired to find what is it in github repositories(((
Please, could you explain what is a truncated index more easier than in white paper? For example N = 200, K = 9