What is a initial rhyme?

[Initial Rhyme] means two completely unrelated verse phenomena (guaranteeing that the term is only ever used by people who don't know what they are talking about).

Firstly it can mean a poem where the rimes come at the beginning of each line, rather than at the end. This virtually never happens in real poetry, but can be set as a class exercise by teachers:

WAIT at the

GATE; get this

STRAIGHT, don't be

LATE!!

....

But sometimes [initial rime] is used as an alternative (and very confusing) way of talking about headrime or alliteration - an older way of writing poems in English where an initial consonant was repeated to structure a line (instead of using standard rime - which became the norm during the Fourteenth Century):

In a Somer Seson whan Softe was the Sonne

I Shope me in Shroudes as I a Sheppe were

Habite like an Heremite unHoly in werkes

And Wenden in the Werld Wondres for to seke

And on a May Morning on Malvern hilles

A Ferly me beFell of Faerie methought (William Langland: Piers Plowman)