Some of the terms used in Google AdWords can be confusing and this is one of them. A view-through conversion is a conversion you have had from the content network in a 30 day period when you are advertising with a display ad. The user did not click your ad, but saw it – confusing right?
First, it used to be that someone had to actually click your ad and visit your website for a conversion to be recorded. Not so anymore, just seeing your ad and visiting a content site will set the cookie that when a user does go later (in a 30 day period) to your website to buy, a conversion is recorded and noted in the AdWords control panel as a view-through Conversion.
It is important to remember that this is only for the Google Content Network and not for Google.com or search partner sites ads and only available for image or display ads. This type of conversion is not recorded for a text ad shown in the content network.
For more in-depth information on view-through conversions visit the AdWords Help Center.