This is the 3073rd most frequent Turkish word.
bekletilen
"Bekletilen" means "that which is kept waiting" or "delayed."
Here, 'bekletilen' acts as a past participle modifying 'mektuplar', describing the state of being delayed or kept waiting.
Bekletilen mektuplar masanın üzerindeydi.
The letters that were kept waiting were on the table.
In this sentence, 'bekletilen' qualifies 'çayı', indicating that it was intentionally made to wait (e.g., to brew).
Bekletilen çayı içmek istedim, ama tadı acılaşmıştı.
I wanted to drink the tea that had been left to wait, but its taste had become bitter.
Here, 'bekletilen' describes the guests, meaning they had been delayed or not attended to promptly.