Tajweed is the science of the correct pronunciation of Qur'anic Arabic.
Tajweed involves learning the details and sounds (phonetics) of the Arabic letters. It allows one to appreciate the subtle differences between those letters that appear to sound the same (e.g. س (seen) vs ص (ṣaad)).
Tajweed in essence, is the art of Qur'anic recitation. The Arabic Qur'an is beautiful - a book like no other. Learning tajweed is very important; it allows you to have flow in reciting, to enjoy reciting the Quran as well as to ensure you are reciting these divine words properly.
Do not fret, tajweed is not hard! It is a simple science that I pray you will learn ameen.
Darul Hijrah has resources here to help you along your tajweed journey.