main street vs high street

main street and high street both are nouns.

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

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

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

main street (noun)	high 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