Tags link related subjects across the site, and make it easier for people to find people and pages that they can help with.
Enter the name of the new tag (e.g. "Oyster Card" - it will be converted to the tag "oyster-card") and it's description in the form box.
Tags can be added and edited by Red Planetters with sufficient Reputation Score.
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.