This is the 7950th most frequent Turkish word.
koyu
"Dark" or "deep" (in color or intensity).
In this sentence, 'koyu' describes a darker shade of a color.
Bu boya koyu mavi renkli.
This paint is dark blue.
Here, 'koyu' is used metaphorically to describe something dense or thick.
Orman çok koyu ve sık ağaçlarla kaplı.
The forest is very dense and filled with thick trees.
In this context, 'koyu' means strong, used to describe tea.