What does it mean to bar a fret?

'Barring' a fret simply means playing multiple notes simultaneously using one finger, in the same fret. To do this we flatten one finger (usually the 1st finger), so that it forms a 'bar'. Have a look at the following image. As you can see, the first finger is covering multiple strings.

Are barre chords hard?

When you first go to play a bar chord you're really at a disadvantage. In that sense bar chords are hard. They're much harder than any of the other chords that you've been playing up to that point. A full bar chord takes roughly equal amounts of strength and proper technique.