What is: "Strut Clunck" mean on a '06 STI ?

Strut clunking on the STI is related to the inverted design they use vs. the conventional monotube struts. They are more prone to getting dirty and making noise, but the inverted design is much stronger than the typical strut. In order to prevent clunking, keep them well greased.

The shock absorber inside the struct unit has likely failed and is moving around "clunking" . Both sides need to be replaced at the same time and the car probably needs to be wheel aligned afterwards. No difficult to do if you have the right tools and are mechanically capable to tackling this large heavy job.