Let's say you have a sigma bond. You have one atom that's connected to another atom that either takes or donates electrons. Think of flourine, it is the most electronegative atom. If it were part of a bond, it would strongly attract electrons. Let's look at an example.
Here, it shows chlorine attracting electrons. Notice that as the carbon gets farther away from the chlorine, its effect weakens. This makes sense. It can't enact as strong of a force from far away. Now, why is this useful?
Carbon donates electrons, and the (+) indicates a lack of electrons. So, the more carbocations the atom is surrounded by, the more stable. However, when you have a negative charge, you want something that takes electrons, like Flourine or something.
Electron Donating Groups
Now, these are useful if you have a positive charge anywhere. That means you don't have enough electrons. In order of strength, they are:
Electron Withdrawing Groups
This is useful for a negative charge. In order of strength, they are: