It means the former.
The rating (12V 5A) of the power supply indicates the voltage it WILL supply (12V) and the maximum current it CAN supply (5A).
It works this way because the supply is voltage regulated, i.e it tries to maintain a constant voltage.
Your load (the lamp) requires 12V and at 12V will draw 2A. 2A is within the capacity of the power supply (5A) so it will supply 12V at 2A to the bulb and everything will be happy.
If you tried to use a 6V 1A bulb, the bulb would be supplied with 12V and would draw more than 1A (probably closer to 2A). The power supply would cope just fine, but the bulb would likely burn out very rapidly.
If you tried to use a 24V 4A bulb, it would be supplied with 12V. At this voltage it would draw less than 4A (let's say it is 2A). Again, the power supply would be happy, but the bulb might just glow dimly.
If you tried to use a 12V 10A bulb, it would be supplied with 12V and try to draw 10A. This exceeds the power supply's rating so one of several things could happen:
1) The bulb gets 12V and 10A and the power supply gets very hot
2) The bulb gets 12V and 10A and the power supply gets very hot and eventually it blows a fuse
3) The bulb gets 12V and 10A and the power supply gets very hot and eventually catches fire
4) The bulb initially gets 12V at 10A, but as the temperature of the power supply rises the voltage is reduced and eventually the bulb is getting something like 3V at 2.5A (the power supply begins to shut down to limit the temperature rise)
5) The bulb gets 12V at 10A for a brief time then the power supply shuts down completely and must be power cycled to start up again
6) The bulb gets 12V at 10A for a brief time then the power supply shuts down completely. After a short time it turns back on and the cycle repeats.
7) The voltage drops immediately to about 4V (or whatever voltage limits the current to 5A
8) Smoke comes out. Then the voltage jumps to 20V, the current spikes and the bulb burns out. Both the power supply and the lamp are dead.
There are many, many more things that can happen. I hope you get the idea though.
In a practical sense, when we use power supplies to test equipment, it's always useful to know what the power supply's behaviour is likely to be.
We have a post in one of the forums containing book reviews. Have a look there and see if anything seems appropriate.
https://www.electronicspoint.com/book-reviews-t251799.html
