This is the 3190th most frequent Polish word.
rzeki
"Rzeki" means "rivers" in English.
Here, 'rzeki' is the plural nominative form of 'rzeka' (river), used as the subject of the sentence.
Rzeki w tej okolicy są bardzo piękne.
The rivers in this area are very beautiful.
In this sentence, 'rzekami' is the plural instrumental case of 'rzeka', used after the preposition 'nad' (over) to indicate relationship.
Zwiedzaliśmy mosty przerzucone nad rzekami.
We visited the bridges spanning over the rivers.
'Rzeki' is again in the plural nominative form of 'rzeka', this time describing the subject undergoing an action (drying up).