Yes, you can use them commercially, and even include them within a product that is sold commercially. Usage and redistribution conditions are specified in the license. The most common license is the SIL Open Font License. Some fonts are under the Apache license or Ubuntu Font License. You can redistribute open source fonts according to those conditions.
Yeah:
https://developers.google.com/fonts/faq#can_i_use_any_font_in_a_commercial_product