This is the 8265th most frequent Russian word.
маршалловых
"Маршалловых" refers to "Marshall" in the context of the Marshall Islands (possessive or descriptive form).
Here, 'Маршалловых' acts as a proper adjectival form of the name 'Marshall Islands' in Russian, showing possession or relation.
Научный экспедиция исследовала коралловые рифы Маршалловых островов.
The scientific expedition studied the coral reefs of the Marshall Islands.
In this case, 'Маршалловых' again denotes the adjectival relation to the 'Marshall Islands', used to modify 'островов' (islands).
Из истории известно, что многие жители Маршалловых островов занимались рыболовством.
It is known from history that many residents of the Marshall Islands were engaged in fishing.
Here, the word 'Маршалловых' modifies 'островах', indicating the geographic relation to the Marshall Islands in locational context.