A T-profile is a long piece, usually metal, whose cross section is shaped like the letter T: a flat flange with a web standing out from it at right angles. It is used in building and metalwork because it is very rigid for its weight.
The key is the shape. A flat sheet bends easily. With the web of the T working edgeways, the piece will not bend one way or the other.
It is the basis of our shutter security system: a 4 mm steel T-profile bonded and screwed to the underside of the bottom slat. Because it is fixed to it, that slat can no longer bend. At both ends there are bolts that lock into the wall.