What is a descriptive phrase (murakkab tawsīfī)?
A noun is named, then a second word describes it. The noun described is the مَنْعُوت (man'ūt); the word describing it is the نَعْت (na't).
The man'ūt is the noun being described; the na't is the word describing it. The na't follows it in definiteness, i'rab, number and gender.
In رَجُلٌ مُجْتَهِدٌ (rajulun mujtahidun), “a hard-working man”, رَجُل is the man'ūt and مُجْتَهِد is the na't. Unlike English, the description comes after the noun.
The na't copies the man'ūt in four things at once: definiteness, i'rab, number and gender. Change the noun and the description changes with it.
Like most Arabic grammar terms, وَصْف (wasf) was taken from everyday speech and then given one precise technical sense.
In everyday Arabic it means a description of something.
وَصَفَ الطَّبِيبُ الدَّوَاءَ (wasafa al-tabību al-dawā') — “the doctor prescribed the medicine”
In Arabic grammar it means a word joined to a noun to describe it.