This tutorial about ethene's bonds has the following paragraph that I hope someone can paraphrase in a better way:
The pi bond dominates the chemistry of ethene. It is very vulnerable to attack - a very negative region of space above and below the plane of the molecule. It is also somewhat distant from the control of the nuclei and so is a weaker bond than the sigma bond joining the two carbons.
What does it mean by:
Here is the meaning of the words ""vulnerable attack" and "negative region of space".
The nucleus of the six atoms (2 C and 4 H) are situated in the same plane, which is usually defined as horizontal. The electrons of the 4 H atoms and the three first electrons from each Carbon atom are all together included in sigma bonds, and their maximum probability of presence is situated in this plan. These bonds are made of negative charges, that should be able to attract positive charges. But these bonds can hardly attract any arriving positively charged ion, because this attraction would be entirely compensated by the repulsion due to the neighboring nuclei.
The pi electrons are very different. Their maximum probability of presence is not situated in the plan. They make a sort of double long cloud high above and much under the plan of the molecule. They produce a sort of negative cloud (or a negative region of space) surrounding the molecule above and under the plan.
Electrons are negatively charged. So they may attract positive charges. If a positive ion like $\ce{H+}$ or $\ce{Br+}$ approaches an ethene molecule, it will be attracted by this negative $\pi$ cloud. If this approach leads to the formation of a new bond between the ethene molecule and the coming ion, it makes sense to state that the coming ion attacks the ethene. One may also state that the $\pi$ cloud attacks the coming ion. In other words, it also makes sense to say that the $\pi$ bond of ethene is vulnerable to attack, at least more vulnerable than the sigma bonds in the plane.
