What is a stub on PCB?
They're used for impedance matching and filtering. 

The image from Wikipedia below shows different stripline features used to create various elements etched into a PCB with descriptions below.




  Some simple planar filter structures are shown in the first column. The second column shows the open-wire equivalent circuit for these structures. The third column is a semi-lumped element approximation where the elements marked K or J are impedance or admittance transformers respectively. The fourth column shows a lumped-element approximation making the further assumption that the impedance transformers are λ/4 transformers. 
  
  (a) A short-circuit stub in parallel with the main line. 
  
  (b) An open-circuit stub in parallel with the main line. 
  
  (c) A short-circuit line coupled to the main line.
  
  (d) Coupled short-circuited lines. 
  
  (e) Coupled open-circuited lines. 
  
  Stripline vias, (the little silver connections on the ends), represent a strap through the board making connection with the ground plane underneath


If you're familiar with transmission lines then you'll recognize the purpose for these elements. If you're not familiar, it's a scary yet fascinating topic I recommend learning more about. It's the weird stuff about electricity. Know your enemy.

There is a bit more explanation in a previous answer I gave on your previous question on via reflections.