Tags link related subjects across the site, and make it easier for people to find people and pages that they can help with.
Tags can be added and edited by Red Planetters with sufficient Reputation Score.
It is quite hard (actually impossible!) to remove a tag at this time, so please make sure that an existing tag does not already exist that more or less covers the topic.
If there is a similar tag already it may make sense to add the keyword you are using to the description of the original tag, as that will make the tag appear in searches anyway.