This is the 12490th most frequent Polish word.
najintensywniejszych
"Of the most intense."
"Najintensywniejszych" here modifies trainings and is in the plural genitive case.
Podczas najintensywniejszych treningów ważne jest, aby się odpowiednio nawadniać.
During the most intense trainings, it is important to stay hydrated.
Here, "najintensywniejszych" is used as a comparative adjective in the plural genitive, describing a set of colors being compared for their intensity.
To jest jeden z najintensywniejszych kolorów, jakie kiedykolwiek widziałem.
This is one of the most intense colors I have ever seen.
In this sentence, "najintensywniejszych" is used as a plural genitive adjective to describe storms during the described states of intensity.