Barre chords, sometimes spelled bar chords, are chords where you hold down more than one string at a time with a single finger. The name probably comes from the same exact barre that you'd imagine ballet dancers practicing and stretching on. Barre chords are very difficult for beginners to play. They require far more tension and balance than an open chord like G major.
This humble blog post can't help you learn guitar barre chords because the topic is complex and nuanced. Instead, this post was written to warn you about them.