This is the 14762nd most frequent German word.
ostfriesischen
The German word "ostfriesischen" means "East Frisian" (relating to East Frisia, a region in northern Germany).
In this sentence, 'ostfriesischen' is an adjective modifying 'Inseln' (islands), describing them as being related to the East Frisia region.
Die ostfriesischen Inseln sind bekannt für ihre Strände.
The East Frisian Islands are known for their beaches.
Here, 'ostfriesischen' describes 'Tee' (tea), indicating its regional origin from East Frisia.
Ich genieße den Geschmack des ostfriesischen Tees.
I enjoy the taste of East Frisian tea.
In this context, 'ostfriesischen' modifies 'Traditionen' (traditions), specifying they pertain to the East Frisian culture.