This is the 4202nd most frequent Polish word.
czynnego
"Active."
The word "czynnego" is used as a genitive singular form of the adjective "czynny" (active, open), describing the pool in terms of its state of being operational.
Nie miałem czasu na korzystanie z czynnego basenu.
I didn't have time to use the open swimming pool.
Here, "czynnego" also means "open" and describes the museum in its operational state, serving attributively.
Ze względu na remont nie dotarliśmy do czynnego muzeum.
Due to renovations, we didn't reach the open museum.
Here, "czynnego" is used in a metaphorical sense to describe activity or activeness as a quality being ascribed.