2-methylbutane: What does it mean?

The IUPAC name "2-methylbutane" contains three pieces of information: "But" means there are four carbons on the main chain of the molecule. "ane" means those four carbons are single-bonded together, and "2-methyl" means there is a one-carbon-long chain attached to the 2nd carbon of the chain.

• "ane" is for alkanes, which are single-bonded together.
  
• "but" means there are four carbons in the main chain
  
• "2-methyl" means there is a methyl group (one carbon long, or CH3) attached to carbon #2 of the chain.

This (below) is butane: Four carbons in a chain, single-bonded together, with hydrogens added so that each carbon has four bonds total.

This (below) is 2-methylbutane: It is the same, but has an extra -CH3 group on the second carbon of the main chain.