What is suction peak?
The suction peak is the point of lowest pressure on an airfoil, in other words the highest point on a typical pressure or Cp graph of an airfoil. On an airfoil it's typically located just after the leading edge on the upper surface, take a look at this image for instance. In that image you can also see that as you increase the angle of attack of the symmetrical airfoil (i.e. as you increase your lift), the suction peak becomes higher. At your stall angle of attack, the "adverse pressure gradient" becomes too high and the flow separates. An adverse pressure gradient means that the pressure increases.

Note that if you would put a symmetrical airfoil at a negative angle of attack, the pressure peak would appear on the lower side, just behind the leading edge.

Also, the suction peak doesn't have to be just behind the leading edge. It is for many airfoils because the point of highest thickness is quite close to the leading edge, but take a look at a natural laminar flow airfoil for instance. The suction peak is around 65%. Here the goal is to keep the flow laminar as far as possible. An adverse pressure gradient will also favour transition of the flow.