Your textbook should be clearer.
The rule is, if there's no external forces on a system, then its centre of mass doesn't accelerate. What's the center of mass? Roughly, it's the average position of the mass of the system. Quantitatively, the center of mass of a system of N particles, each with mass m_{i} and position r_{i}, is
[tex]\vec{r}_{\mathrm{centre \ of \ mass}}=\frac{1}{M}\sum_{i=1}^N m_i \vec{r}_i.[/tex]
