In the Main information of your campaign, you will find two Tag sections.
The first one is called Main category and is located in the upper section of your form. It defines the category in which your campaign will be displayed.
The second is the Tags section, and it contains a list of predefined keywords. You can select up to three tags. It will help your campaign to be displayed in search results if someone does a search on these keywords.
Also, keep in mind that the best way to optimize your SEO is to write the content of your page using different words, all related to your project.