In a court room, they use a different language than outside. It sounds like English but the words don't mean what we have been led to believe. What "not guilty", means is more like saying, "I'm guilty but I want you to go through the effort of proving it." It is like a light switch that is off. It is still a light switch, even if you call it an off light switch.
Another they use is "Public Defender". We've been led to think that their job is to defend us. Yet right in their title they tell use who they are defending. They are really there to protect the Public from our dishonor.
Asking for a plea is really asking, "how do you want to play this game that we've offered you?" Any way that you answer is playing along with the court. The time to declare that you don't want to play is long before ever being charged.
AND, admitting to understanding the charges is the same as agreeing to stand under the charges.
If you weren't guilty, you'd plead innocent.