Phonology is the study of the smallest contrastive units of language. These units are the smallest building blocks of language. The building blocks are organized to create words.
Linguists study how the units are organized in a language, how they are combined to form a word, and how they interact with each other.
The smallest units of language in spoken language are called phonemes. In signed language, each sign consists of the five basic parts, called parameters, in which each of the five parameters has its set of primes.
Each ASL sign (or word) consists of all these parameters: handshape, movement, location, palm orientation, and non-manual signals/markers. These parameters were originally described by Dr. William Stokoe. Later, the NMS was added by other linguists.
These parameters (primes) are building blocks to create a word or a morpheme. Without any one of these parameters, there would no signed word.
Each paramater has a number of primes. For example, the handshape paramater has over 40 different handshapes (handshape primes).
A signed word results in a different meaning when one of these paramaters (or primes) changes. If a word is pronounced incorrectly, it may distort its meaning like "misspelling" or sometimes meaningless.
Many ASL words have more than one prime of a parameter in a single production. For example, the ASL word mother+father ("parents") has one handshape, one movement, one palm orientation, and two locations. Another example, the ASL word good has one handshape, one movement (or sometimes two), one palm orientation, and two locations.
Clayton Valli, et al. Linguistics of American Sign Language: An Introduction. p 17.