high street vs main street

high street and main street both are nouns.

Word	Noun	Adjective	Verb	Adverb
high street	Yes	No	No	No
main street	Yes	No	No	No

As nouns, high street and main street are synonyms defined as:

high street and main street: street that serves as a principal thoroughfare for traffic in a town

high street (noun)	main street (noun)
street that serves as a principal thoroughfare for traffic in a town	street that serves as a principal thoroughfare for traffic in a town