Tajweed is a set of rules for proper pronunciation and recital of the Quran. It is meant to replicate the way the prophet Muhammad (saws) recited the Quran. Learning tajweed means learning how to pronounce the letters in the words correctly, which covers matters like when a consonant is silent or not and how long a vowel is kept.

It is only meant for reciting the Quran however, and does not apply to enunciating (classical) Arabic.