An adjective is a part of speech that modifies a noun or pronoun. Adjectives are often confused with adverbs which can modify pretty much everything else. Some examples:
“The big tree stood in the front yard.”
“Dogs are very fond of yellow fire hydrants.”
“The keys are on the oak table.”
Possessive pronouns (my, his, her, etc) can also be used as adjectives in certain situations. For example:
“I would like to share my candy with you.”
In some cases, certain words (this, that, which, what, etc) can also be used as adjectives. Examples:
“This book is very good.”
“Which pony do you want for your birthday?”