A Clickthrough is usually used in an advert. It is the process of clicking on an advert which then takes you straight through to the destination that was advertised.
For example, if you saw an advert for one of are articles saying, “Click here to find out all about XXX”. A clickthrough would mean when you clicked on it, it would take you straight to the article. Or another example if there was a 15% discount on all shoes on my website and you clicked on my advert. A Clickthrough would have you taken straight to my site with the 15% discount applied.