I'm sure that TL has other meanings, but here is my definition:
Loudspeakers where the enclosure takes the form of a long tube. The woofer is mounted in one end, the other end of the line is open. The tube is usually tapered, and contains stuffing. The degree of taper effects the bass output, as does the density of the stuffing, since it absorbs some of the sound passing through it, cuts down resonances in the line, and also changes the speed of sound within the line. If these are not optimised, there will be too much mushy bass, or well controlled but weak bass. An extra complication is that the pipe will often be folded, to make the speaker a more domestically acceptable size.
What's the point?
Part of it is to do something useful with the back wave of the speaker. The long, stuffed line absorbs the high frequencies, and the line length and amount of stuffing is set so that the bass which exits the line reinforces that coming from the front of the speaker (similar to a bass reflex design).
Why not just use a bass reflex design?
The air behind the driver in a TL is very loosely enclosed, it does not 'load' the driver into a resonant system the way bass reflex or sealed enclosures do. This makes the TL's an excellent design in terms of transient response, whereas bass reflex speakers are one of the worst of the enclosure types in this regard. Also, the low frequency roll-off of TL's is very gentle, rolling off much more slowly than the bass reflex sharp cut-off.