un primo = a cousin (male)
una prima = a cousin (female)
unos primos = "cousins" or literally "some cousins" (male or male and female mixed, but more than one)
unas primas = "cousins" or literally "some cousins" (all female only)
In the dictionary, if you look for the word "cousin" you will find the masculine singular form = el primo ("the" cousin)
That also has 4 forms = el primo, la prima, los primos, las primas. Can you guess the number (singular/plural) and gender (masculine/feminine) of each?