\textbf{The "Chirp" Signal Explained} 

Gravitational waves are produced when mass is accelerated, but their amplitude is generally super tiny. However when two massive, compact objects rotate very close to each other (for example two neutron stars or two black holes) then we have both very large masses and huge accelerations, the key ingredients for producing high amplitude gravitational waves. A very specific gravitational wave signal called a "chirp" is expected when two such compact objects merge. 
Let's look at the case of two black holes, as for the LIGO signal. First the two black holes orbit each other and go through an inspiral phase in which the orbit shrinks due to increasing energy loss into gravitational waves. Smaller orbits correspond to higher orbital speeds, with the black holes moving faster and faster (up to about 50\% the speed of light in the case of the system observed by LIGO, see figure below) and the frequency of the generated gravitational waves increasing accordingly. The amplitude of the gravitational waves increases too, until the two black holes coalesce and the gravitational waves emission reaches a peak. After that the newly formed, more massive black hole goes through a sort of adjustment phase called ring-down. The event emits large amounts of gravitational waves only during the very last second before the coalescence occurs. Most importantly, the mass of the black holes, their distance and other important parameters can be reconstructed by looking at the  particular properties of this very distinctive chirp signal. Sweet!