When the engine is running it does sound like it would normally, it can sound like it's struggling to keep running or produce strange noises that sound like it's not running in time with itself.
The car has failed the part of the MOT test that checks the emissions levels coming from the car's exhaust. The car cannot get an MOT certificate until the emissions levels are at the acceptable level.
When pressing the accelerator down the car does not accelerate as it's supposed to, there is sometimes a lag or delay between pressing the pedal and the car accelerating. The acceleration is sometimes slower than it should be and the car can feel as if it's hitting a limit that stops it going faster.