Understanding math is important to a strong educational foundation. A great book to read about how and why to build a good educational foundation in math is Liping Ma's "Knowing and Teaching Elementary Math."
Mathematics Enhancement Programme (MEP) is a free online math curriculum that can help build a strong math foundation. You can download and print the materials for free at the Centre for Innovation in Mathematics Teaching thanks to several charitable foundations.
Don Potter's math page has links to variety of excellent free math resources.
Kahn Academy has math videos and online exercises for all levels of mathematical abilities.
Living Math is a fun way to sneak in a bit of math understanding while developing an appreciation for math and making math fun.