This is a graphical representation of the space spanned by a two qubit quantum computer. There are four basis states for two qubits, $\left|00\right>$, $\left|01\right>$, $\left|10\right>$, and $\left|00\right>$. These represent the four possible observed values of the two bits. According to quantum mechanics, each of these four values has both an amplitude and a phase. The amplitude has the property that when squared it is the probability for that state. The phase has a meaning that depends on the type of experiment you carry out but for the purposes of this representation is simply an angle between 0 and 360 degrees.