What does this mean?
Brake imbalance is caused because one side of the braking pair (front or back axle) is not applying as much power as the other side.
What's wrong with my brakes?
There are too many answers for this forum. It could be any number of things like a sticking caliper, or a worn pad, or air in the lines.
What does the 23% mean?
When the braking force for each side is measured, one side has 23% more stopping power than the other. I believe for the MOT it must be within 25% in order to pass.
How can I fix it?
It all depends on what the problem is and what type of brakes are being used. Without more information, like your brake type and what mechanically has been done recently, it would be hard to give you a good answer here.
Will it affect the braking of the car?
Yes it does affect the braking of your car. At 23% you can probably feel it pull to which ever side is stronger, but not bad enough you'd lose control of the vehicle. If you have disk brakes there is little you can do about it but check to ensure the calipers are working correctly, you have good and even pads, the rotors aren't worn out, and that the calipers slide correctly.
Will it be okay if I just leave it?
If you are happy and don't feel there is any issue, you can leave it. MOT says it's just an advisory, so no worries there (this year). Personally, I'd want the bias down below 10%, so would try to figure out what the issue might be.