Khabarovsk is one of the major cities in the Russian Far East.
Literal
Khabarovsk-city [subject-が] Far-East Russia [at-で] big-city [genitive-の] one [copula-です].
~のひとつ ('one of the X') is the standard 'X is one of Y' construction. Note the use of が rather than は for the subject — this is focus が, presenting Khabarovsk as the answer to 'what is one of the major cities.' 極東ロシア is a compound of 極東 ('Far East') + ロシア ('Russia'). Common pattern in encyclopedic writing.