D flip flop data extraction
It just means that Q only changes (to D) on the rising (or falling in some FFs) edge. At all other times it will remain in the state clocked from the last rising (or falling) edge, even if D changes.  

So you can think of it as capturing the value of D at specfic moments in time (e.g. on the rising clock edge), and storing this value until the next rising edge (or replace "rising" with "falling" for a negative edge triggered FF)  

This is assuming a D flip flop as linked to in the question.