In the early days of JIT (just in time) manufacturing there was a transfer from storing large quantities of parts you needed to make your widget, to only keeping a few days worth on hand, and generally a few days worth in trucks on the road coming to you. Theoretically, in a perfect world you always had parts coming in the back door and widgets going out the front door. If there was a hiccup in that supply chain, parts would have to be expedited to fill that gap.
In comes the 'hot shot' drivers that had smaller trucks capable of holding generally half or less than an over the road truck, considered LTL or less-than-truckload freight. The smaller trucks were easier to find drivers for as they didn't require quite the classification requirements for drivers licenses. There was a boon of them in the late 90's to fill the JIT needs for expediters of freight. Now the name applies to any small time operator, usually with a single truck and a gooseneck trailer hauling the stuff that wouldn't necessarily fill an over the road semi, or generally doesn't pay enough to be worth their time.
Anyone that was driving over the road in the mid to late 90's should remember the explosion in the numbers of Panther 24' LTL trucks that happened seemingly overnight. That was the primary company at the time filling the LTL gap for JIT trucking.