What is a spanish adjective agreement?

I think you mean that Spanish nouns having gender and number, this has to be shown for corresponding adjectives, so:

mujer hermosa (beautiful woman)

'hermosa' (beautiful) ends in 'a' because feminine singular

mujeres hermosas (beautiful women)

ends in '-es' (noun) '-as' (adjective) because feminine plural

hombre hermoso (handsome man)

'hermoso' (handsome) ends in 'o' because masculine singular

hombres hermosos (handsome men)

ends in '-es' (noun) '-os' (adjective) because masculine plural

NB ALL nouns are masculine or feminine in Spanish; gender is not sex.