This means that some N-t063 words are written without yod and some N-t063 words are written with a yod after the first root consonant.
For instance:
the word
תֵּבֵל | תבל meaning 'world' (see last word in 1Sam 2:8) is written
תבל
But the word בֵּרֵךְ, a verbal form meaning 'he blessed' (we find in Psa 10:3) will be written with a yod after the first root consonant:בירך
You will find the explanation to this in another place of Oham.