A loose coupler is where the primary coil can be varied in coupling to the secondary coil by sliding it in and out. That whole cylinder with the taps on the end can go in and out. A fixed coil is in the box, and the coupling is variable by pulling the secondary coil out. The looser the coupling, the sharper the selectivity. So, if you have very tight coupling, that is, with the coil jammed all the way in, it would be loud, but it would not be selective for a particular frequency.
Text from the transcript of a tour of New England Wireless & Steam Museum’s Wireless Building given by Robert W. Merriam on a winter day in 2012. Transcription by Craig H. Moody, K1CHM. Edited by Fred Jaggi.