“Let no man therefore judge you in meat, or in drink, or in respect of an holyday, or of the new moon, or of the sabbath days: Which are a shadow of things to come; but the body is of Christ.”

Here, a shadow is defined as an image cast by an object and representing the form of that object. Actually, it is opposed to σῶμα (body), which is speaking of Jesus Christ. Follow the thought process:

After reprimanding the early Jewish believers because they were “judging” the Gentile Christians for not practicing the Jewish “feasts and festivals,” along with worshipping on the “sabbaths,” Paul explains that all of those were just “shadows” of things to come.

This is like when you look at a PHOTOGRAPH of a person; it is just a shadow of what they looked like at the time. But supposing the person in the photograph showed up at your front door as you opened it, that would be the REAL THING! JESUS CHRIST is the REAL THING!!