This is the 13869th most frequent Turkish word.
mahalleyi
"The neighborhood" (accusative case).
In this sentence, 'mahalleyi' refers to the direct object, specifying the particular neighborhood that Ahmet visited.
Ahmet mahalleyi ziyaret etti.
Ahmet visited the neighborhood.
Here, 'mahalleyi' is the direct object indicating which neighborhood was liked.
Mahalleyi çok beğendim.
I really liked the neighborhood.
In this context, 'mahalleyi' functions as the direct object showing the neighborhood that was left.