When playing guitar (depending on whether you are a righty or a lefty), you will use one hand to pick/strum the strings and the other hand to form chords, play leads, etc...

So you have one "picking-hand" and one "fretting-hand".

Your picking-hand is used to either...

1. pick/strum the strings using a guitar "pick" (also called a plectrum)

2. pick the strings using just your fingers/fingernails.

You fretting hand is used to hold down strings on the fretboard.

When you pluck or strike a string without "fretting" it, that's known as playing the string "open".

This is also known as playing an "open string".