The term root refers to the note which gives the chord its name. For example, A is the root of A7, and E is the root of Em.
You have learned three types of chords, built on еach of the seven roots, That's our twenty-one chords. Learn and memorize where the root is located on each chord shape. Most of the time, the root is the lowest note (also known as the „bass note“). You should be able to clearly pluck the root first, and then follow that with full strumming. The coordination should feel comfortable at many different tempos. Remember to vary your tempos and your strums.