This is the 7134th most frequent Russian word.
брестского
The Russian word "брестского" means "of Brest" or "Brest’s," referring to something related to the city of Brest.
Here, 'брестского' functions as an adjective describing the treaty related to Brest.
История Брестского договора изучается в школах.
The history of the Brest Treaty is studied in schools.
In this sentence, 'брестского' acts as an adjective in genitive case, describing something belonging to or associated with Brest.
Брестского крепостного музея посетило множество туристов.
Many tourists visited Brest's fortress museum.
In this usage, 'брестского' describes 'периода' (period), indicating a time associated with the city of Brest.