Tags are pieces of code coming for other online tools like Google Analytics, Xiti, etc. They can be copied and pasted in their integrality in this window to load them in the source code of your campaign.
Some of them are specific and are only loaded on the first page of your campaign to avoid double tags. Other users use this tab to place additional HTML code in order to add specific items to their campaigns. [Banners, frames under the campaign etc.]