What does programming an ECU mean?
The ECU is the computer that runs the engine. Programming it means loading the manufacturer's software plus the data for THAT specific car — VIN, codes, configuration — so the other modules recognise it as theirs and the engine can start.
ECU stands for engine control unit. It is the module that decides how much fuel goes in, when the spark fires and what the turbo does, reading dozens of sensors several times a second. When it is replaced, the new one arrives blank: it knows how to be an ECU, but not how to be YOURS.
Programming has two halves that often get confused. The first is loading the right software for that engine and that year, which the manufacturer publishes and which gets downloaded. The second, the one that actually takes work, is the marriage: writing in the VIN, syncing it with the immobiliser module and with the transmission module, and getting all three to accept each other. If that part fails, the car cranks and never catches.
The TCM is the same story for the automatic transmission, and the two are usually done together because they are synced to each other. That is why you will almost always see "ECU and TCM" as a pair.
This is not the same as "remapping for more power", which is a different job with a different name. This is putting the car back to how it left the factory, with a new part inside it.
Where this comes from
Also in the glossary
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