Adjectives are words that inflect for gender as well as case and number. Adjectives are said to agree with the nouns they modify. This means that when you modify a noun with an adjective you must choose from among three gender forms of the adjective so as to match the gender of the noun.
masculine: Јто мћй стакђн. That's my glass.
  Јто мћй пђпа. That's my Dad.
feminine: Јто мої рџчка. That's my pen.
  Јто мої мђма. That's my Mom.
neuter: Јто моё письмћ. That's my letter.
An adjective must agree with the noun with respect to case, too. Here are examples of an adjective agreeing with nouns in the Accusative case:
masculine: Ћн взїл мћй стакђн. He took my glass.
feminine: Ћн взїл моє рџчку. He took my pen.
neuter: Ћн взїл моё письмћ. He took my letter.