Traditionally, we understand this to be a colorful coat made of fine and expensive threads. This is one possible description. The Hebrew wording is very unusual and difficult to really interpret. The word describing the tunic is only found in one other place in all of the Hebrew scriptures and its meaning is not really known. In that other instance (2 Samuel 13) it is understood to mean long. So what does this say about Joseph’s coat? Was it colorful? Was it made of fine and expensive fabrics? Was it unusually long? And why would any of these be reason to make others jealous? Remember the story of Noah and Ham? One understanding of the situation with Joseph’s coat is that it was unusually long, indicating Jacob’s intent to give a very large share of his inheritance to the younger of his sons. I can’t say this is the definitive answer, but it would make a lot of sense.