Adding tags to your campaigns can be useful for grouping together campaigns for reporting or analysis - for example you might have a number of campaigns that are raising money for the same project - you could create a tag for the project, tag all the relevant campaigns with the project tag and when you create a list you could filter for campaigns with the project tag.
Firstly you need to set up the tags - usually it is best to create a new tag category for your campaign tags - this article explains how to set up tags - you need to make sure your Tag Category can be applied to Campaigns - see screen shot below
Any campaign tags you have setup will be shown when you click on Campaigns in the menu - you can filter the campaigns by selecting the tags - as shown below
On the campaign profile tab you can select the tags you want to apply - see example below
If you create a campaign list you can add a filter for active or excluded tags - see example below