This is the 893rd most frequent Finnish word.
olevaa
"Being" or "existing."
In this sentence, "olevaa" describes the decorative pattern that exists (is present) on the dog's leash.
Koiran hihnassa olevaa koristekuviota ei ollut helppo havaita.
The decorative pattern on the dog's leash was not easy to notice.
Here, "olevaa" refers to the book that is said to be present in the library (but isn't).
Kirjastossa olevaa kirjaa ei löytynyt hyllyltä.
The book supposedly in the library was not found on the shelf.
In this case, "olevaa" is defining the coffee cup that exists (is present) on the table.