Click on buy and you should be asked for a serial number. Enter your number and it will convert to Photoshop Standard.
actually i misspoke, it says license or trial. but your tip worked. so what is it for?
When you first install Photoshop, unless you input a serial number or sign in to a Creative Cloud subscription, it will default to a 30 day Photoshop Extended trial with an ad to join Creative Cloud.
A couple of weeks ago I opened my licensed copy of Illustrator CS6 on my computer after having not used it in a while. I know creative cloud verifies the subscription every 30-99 days by making you sign in, so even for licensed users I suppose there is a check like that as well.
If forgot whether the term was "License" or "Buy", but I do know you will be asked to sign in and supply your serial number.
As a final caution, be sure your Photoshop serial number is registered with Adobe. You can then access it from your account anywhere. I don't know what to tell someone who didn't and the package or receipt is buried in a storage locker, or got thrown out and they have a project for a client waiting.