What part of speech does "s" stand for in WordNet synsets

To add to the technical detail tripleee shared, following gives better understanding of what satellite adjectives are -

Certain adjectives bind minimal meaning. e.g. "dry", "good", &tc. Each of these is the center of an adjective synset in WN.

Adjective satellites imposes additional commitments on top of the meaning of the central adjective, e.g. "arid" = "dry" + a particular context (i.e. climates)

Source: https://www.englishforums.com/English/AdjectiveSatellite/nwzhv/post.htm

https://wordnet.princeton.edu/documentation/wndb5wn documents this as ADJECTIVE SATELLITE. https://wordnet.princeton.edu/documentation/wngloss7wn explains that adjectives are subcategorized into "head" and "satellite" synsets within an "adjective cluster":

Adjectives are arranged in clusters containing head synsets and satellite synsets. Each cluster is organized around antonymous pairs (and occasionally antonymous triplets). The antonymous pairs (or triplets) are indicated in the head synsets of a cluster. Most head synsets have one or more satellite synsets, each of which represents a concept that is similar in meaning to the concept represented by the head synset. One way to think of the adjective cluster organization is to visualize a wheel, with a head synset as the hub and satellite synsets as the spokes. Two or more wheels are logically connected via antonymy, which can be thought of as an axle between the wheels.