Translation:This is the tallest building in the city.
I think it a way of turning an ordinary adjective into a superlative. In English, we can usually do that by either adding an est onto the adjective, or putting a most in front of it. We usually only use the most form for multi-syllable adjectives in English (we say biggest rather than most big, but say most gigantic rather than giganticest). So (as a non-Russian) I'm imagining it plays the role of most.
ah ok thanks. this can be confusing sometimes because my native language is Slovenian. the escalation of adjectives is structured differently. for instance, tall = visok and tallest = najvišji. thats why it's hard for me to get used to adding another word to escalate the adjective such as "самое"
We also have наивысший but it is even more of a bookish/poetic word ("Напряжение достигает наивысшей точки"). Высший in general covers the more figurative meaning of "high", the one you see in высшее учебное заведение or высшая математика.
Наивысочайший also exists but it is fairly rare, with 7 instances in the 320-million corpus.
It is the same with other adjectives: we have длиннейший, нижайший, широчайший, крупнейший, популярнейший and even наидлиннейший, наинижайший, наиширочайший, наикрупнейший, наипопулярнейший—we just do not cover them in the course. A few of them like наибольший and наименьший are at least common (well, not in speech).