1A1 is a wheel shape/type designation (from common abrasive wheel nomenclature). It generally refers to a flat disc / straight wheel profile (i.e., a simple, face-type wheel rather than a cup or dish shape).
A resin-bond diamond wheel means the diamond grains are bonded in a resin (polymer) matrix, which tends to be more flexible/cushioned than many metal bonds.

In short: a 1A1 resin-bond diamond wheel is a flat/straight diamond wheel (disc type) with diamond held by a resin bond.

Typical applications
Because it’s a flat disc and resin-bond wheels are often used for precision finishing, 1A1 resin-bond wheels are commonly used for:

Surface grinding / face grinding of hard materials
Precision grinding and finishing where you want good surface quality
Light to medium stock removal on materials such as:
carbide / tungsten carbide