This is the 305th most frequent Russian word.
этот
"This" or "that" (masculine, singular).
Here, 'этот' is used as a demonstrative pronoun pointing to a specific object, in this case, 'дом' (house).
Этот дом очень красивый.
This house is very beautiful.
In this sentence, 'этой' is the declined form of 'этот', used to indicate the specific street ('улице').
На этой улице много людей.
There are many people on this street.
Here, 'этой' again is the declined form of 'этот', agreeing in case, number, and gender with the noun 'книге' (book).