Imagine I have three points $p_1$, $p_2$, $p_3$ in $3$ dimensional space. Imagine I also have two points $c_1$ and $c_2$ in the same space. how to determine if $c_1$ and $c_2$ are on the different sides of $p_1p_2p_3$ plane ?
First you find the plane passing through the three points p1,p2 and p3. It can be done by taking the normal to the plane to be $(p1-p2)\times (p1-p3) = n$. Note that the normal $n$ lies on one of the two sides. Then you find the two vectors connecting the plane to $c1$ and $c2$, i.e. $p1-c1$ and $p1-c2$. At this point the scalar product with the normal indicates how much angle is in between the normal and the two directions. Therefore, if $n\cdot (p1-c1)$ and $n\cdot (p1-c2)$ have different signs, they lie on different sides of the plane.