Read the manual to your fan.

The ION button on your fan is using a high voltage to electrically charge air particles. It has been shown that there is "greater subjective well-being in an artificially negatively-ionized environment", but the reason for this is not clear.

However, it has been shown that most all commercially available air ionizers do not ionize enough air flowing through them to have the desired effect in most large areas. So, the ION button will attempt to ionize the air flowing through it, but unless you are standing directly in front of the fan, you likely won't really experience any effects.

The positive effects of ionization are largely 'subjective' at this point, so it can vary from person to person how ionization affects you.