A contextual link is where the clickable text is made up of a keyword phrase. Since you are advertising a link building service (which I presume is yours) I'm shocked you don't know this.
A contextual link is one in which the clickable text with a keyword phrase it point to the client website.
Another definition: Contextual links are those links which are coming from content/blog post/articles with your anchor text.
An example would be: if you're trying to rank for the keyword free ipad, you can see "free ipad" is the anchor text for the site http://freelaptopsavailable.com.