A call to action is a marketing term that refers to the action that a marketer (in this case site owner) wants the client (or visitor) to perform when viewing the advertisement (or visiting the page).

A call to action could be any of the following:

1. Click a link to visit another page (i.e. reduce the site bounce rate)
2. Enter a search to find a suitable product
3. Click a link to sign up on the site
4. Click a button to book a ticket or purchase a product
5. Call a telephone number listed on the site

Every web page on a site should have a call to action and the call to action should be relevant to the purpose of the page.