What does an apostille do?

An apostille does not make any changes to the content of the document; rather it certifies that a specific person or organization has verified that this document is authentic, unaltered and valid for legal purposes.

For example, let's say your friend has recently gotten married in Spain and wants you to sign his wedding certificate as a witness. Your friend could take this document to your local county clerk's office and ask them to verify its authenticity before they accept it as real evidence of his marriage license. However, if he wanted you to send it back to him in Spain so he could use it as proof of his marriage there as well, he would need some kind of certification from the US government that this document was actually from America—and not just a copy of an original made in Spain by someone impersonating your friend (or someone who just happens to look like him).

This is where the apostille comes in: It's essentially another layer of security on top of whatever other authentication method you choose (for example, using a notary public).