Your left and right hands have the same fingers in the same order. Still, a left glove only fits a left hand. Some molecules come in a left-handed and a right-handed form in just the same way.
Hold your left hand up to a mirror: the reflection is a right hand. The two hands are mirror images. Lay one hand on top of the other, both palms down, and the thumbs point opposite ways. However you turn them, they never match exactly.
Anything that differs from its mirror image is called chiral, from the Greek word for hand. Gloves, shoes and corkscrews are chiral. A plain mug, a spoon and a football each match their own mirror image.
Molecules are 3D objects, so they can be chiral too. A chiral molecule comes as a pair of enantiomers: two hands, each the mirror image of the other. Both hands have the same atoms, joined in the same order. Isomers like butane and isobutane have their atoms joined in a different order. In a pair of enantiomers, only the arrangement in 3D is reversed, the way your thumb sits on the other side of a right hand.