Adjectives are special words for describing nouns.
Хороший человек – nice person
Отличная погода – good weather
The same adjective may have 3 variants of endings:
красивый
красивая
красивое
красивые
All these 4 words mean “beautiful“. So you may ask, what’s the difference between them. The difference is only that Russian adjectives always match with nouns they’re describing. If you’re describing a masculine gender noun you need to use a masculine ending for adjectives (-ый / -ой / -ий). For describing feminine nouns you need to use feminine endings for adjectives (-ая /-яя).
Красивый мужчина – a handsome man
Красивая девушка – a beautiful girl
For example:
телефон (он) – a telephone (he)
машина (она) – a car (she)
окно (оно) – a window (it)
And when we have an adjective to describe a noun it has to be matched with the noun. It will be:
красивый телефон – a beautiful phone (he)
красивая машина – a beautiful car (she)
красивое окно – a beautiful window (it)