What is the meaning of such terms as general and special words and frequent and rare words?
Linguists can “rate” and taxonomize “signifiers” by their recognizability. Thus, open class words (nouns, verbs, adjectives, adverbs) that have less specialized application are “general” while words that are more specialized are specific. An example is the general word "gallop" opposed to the specific word "canter": most children know horses gallop, yet it takes specialized knowledge to know horses also canter and to know what a canter is.
Words that occur quite often in common vocabulary use, like "house", "can", "please", "run", "pretty" are frequent words. These frequent words are often from Anglo-Saxon Old English origin. Rare words, on the other hand, are seldom used or even little known words, like "prolix," for example, instead of the frequent word "talkative." This might also be extended to a frequent word with a rare meaning such as “grave” as an adjective as in "words too are grave.”
The main importance of general and specific words and frequent and rare words in linguistics is in relation to vocabulary retention in response to the question of attrition and rates of attrition, answering "which elements of lexicon are most likely to be lost" (Abbasian, Azad University).