Hiding default affiliate links that company gives you increases clicks and in this way you can easily monetize your WordPress site.
Masking and hiding affiliate links actually means that you can hide somedomain.com/products/links with yourdomain.com/links. This is just an example you can do more.
So for starting up with affiliate link masking first of all install and activate Pretty Link Lite, then visit Dashboard -> Pretty Link and click Add New Link button. Now add the Target URL, Permalink, Title and select the Redirection Type from Temporary and Permanent redirects. You can also set parameters, no follow attribute, tracking hits option etc.