What is a V-I cadence?

A V-I cadence is a perfect cadence or a cadence that sounds finished. Many works finish with a perfect cadence. If you're in the key of C major you count the fifth note in the scale which is a G and create a G major triad which is GBD. Then to get the I, you find the first note in a C major scale which is C create a major triad CEG and then play these chords side by side and they create a perfect cadence.