The torque converter is a fan that spins the transmission fluid. The spinning transmission fluid forces another fan to turn which runs the transmission. That fluid coupling is what allows the engine to stay running in gear when the car isn't moving. There is always about 500 to 700 rpm drop in rotational speed in the second fan which results in a loss of fuel mileage. Starting with the '77 models, Chrysler developed a "lockup" torque converter with a clutch that engages in third gear, above 35 - 40 mph. That increases fuel mileage. It can feel like another shift when it engages. All other manufacturers are using a similar design.
The converter will unlock when you tap the brake pedal or when you fully release the gas pedal to coast. On newer cars that lockup clutch is computer-controlled so there's a number of things that can prevent it from locking up. The engine has to be warmed up too. When it doesn't lock up, the mechanic has to determine if the computer commanded it to lock up but it didn't respond or isn't working, or if the computer did not request lockup, and for what reason. You can drive the car that way but your fuel mileage will be down perhaps 20 percent. It sounds like your computer saw that it didn't lock up when commanded to do so.