This is the 599th most frequent Turkish word.
mahallesi
"Mahallesi" means "neighborhood" or "district" in English.
In this sentence, 'mahallesi' means 'his/her neighborhood' and indicates possession using the personal suffixes in Turkish.
Mehmet'in mahallesi çok güvenlidir.
Mehmet's neighborhood is very safe.
'Mahallede' here means 'in the neighborhood,' using the locative suffix '-de' to indicate location within the neighborhood.
Bizim mahallede her hafta pazartesi pazar kuruluyor.
In our neighborhood, a market is held every Monday.
In this usage, 'mahalle' refers to 'neighborhood' generally and is in its base form to denote the subject of the sentence.