This is the 17775th most frequent Russian word.
ультрафиолета
"Ultraviolet" (a type of electromagnetic radiation).
Here, "ультрафиолета" is in the genitive case, which is required by the construction "использование ... помогает" (use of ... helps).
Использование ультрафиолета помогает дезинфицировать воду.
The use of ultraviolet helps disinfect water.
In this sentence, "ультрафиолета" is in the genitive case, following "свет", which means "light".
Кварцевые лампы производят свет ультрафиолета для стерилизации.
Quartz lamps emit ultraviolet light for sterilization.
Here, "ультрафиолета" is in the genitive case after "количество", meaning "amount".