What should I bear in mind when using Google Fonts licensed under the Open Font License?

The following key aspects should be considered:

• Fonts under this licence may be used freely and also in a commercial environment, e.g. in products, software or documents that you wish to distribute

• Fonts under this licence may be used in software, but must then be supplemented by a copyright notice (to be found at the top of the attached terms of use) and a copy of the terms of use of the software

• Fonts under this licence may be modified and redistributed, but only under the same licence (Open Font License)

Source:
SIL Open Font License Version 1.1