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 Campaign 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 from with Settings > Configuration > Tags & Tag Categories.
This article explains how to set up tags - you need to make sure your Tag Category can be applied to Campaigns - see screenshot below
On the campaign profile tab, under the Transactions section you can see the Tags area, you can select the tags you want to apply from within this area - see example below:
Any campaign tags you have applied to your campaigns will be shown when you click on Campaigns in the menu - you can filter the campaigns by selecting the tags - as shown below:
If you create a campaign list you can add a filter for active or excluded tags - see example below: