In order to complete the preceeding answers, I would say that "strength 5" is part of a code used between pilots and air traffic controllers to describe to quality of radio transmissions and receptions.
The rating goes from 1 to 5, the latter being the best quality one. We don't usually give that information in every single transmission. In this particular example given by Hkg_clk it's impossible to find out who's calling who but definetely it is at least the second time the call has been made without getting answered: Let's say the ATC has called the pilot to give him his taxiing clearance and didn't get any answer. Then, the ATC gives the pilot his clearance again asking how this one was receiving. The answer from the pilot would be "Strength 5" and a read back of the clearance given.
G-CD is the registration of the aircraft and if the ATC used the proper phraseology, G would be the first letter and CD, the last two ones. In this case, we are talking about a plane registered in the UK (G as first lettre).
To make a difference with the runways' designations, which always are two figures (giving the orientation of the runway) and to avoid any confusion from the pilots between runways and taxiways, the latter are exclusively named with either a lettre (Alpha, Bravo....) or a combination of lettre and figure(s) depending on the importance of the airport.
For instance: Taxiway Alpha, Alpha1, Alpha12....
Now that you have all the details, here is the translation of the transmission given by Hkg_clk:
" G-CD, (receiving you) strength 5, Taxi via (taxiway) C and hold at (holding point on taxiway) B4 for (a take-off runway) 04.
Sorry for being a bit long on this matter.:o
The sky has no limit...