Yellow lines are placed on the road because if vehicles were permitted to park there it would cause an obstruction of some sort so by ignoring the single yellow lines and parking there you are putting yourself and other motorists and vehicles at danger.
There are restrictions on each specific sign that will identify the times when you cannot park there, generally 0800hrs -1800hrs but check the sign prior to parking.
You could face a fine if you park illegally or you could even have your car towed away.