What is Mafic; intermediate; felsic?

A magma, or the rock it forms, is mafic when it holds about $45\,\%$ to $55\,\%$ $\ce{SiO2}$ and is rich in iron, magnesium and calcium; intermediate between $55\,\%$ and $65\,\%$; felsic when it holds about $65\,\%$ to $75\,\%$ $\ce{SiO2}$ and is poor in iron, magnesium and calcium but rich in sodium and potassium.