Low
That red line is at 70, and anything below that is considered low and dangerous - however non Type 1s sit in the 60s regularly, so if I am just hanging out between 65 and 70 I don't usually worry about it unless I feel off or am trending downward.
High
The yellow line is at 200 (I've since lowered it to 160) and anything above that is considered high. Most non-diabetics will never even come close to these numbers. I've spent days above them. I wasn't happy about it, but sometimes you do everything you can, and you're just high.
Normal
A non-diabetic's normal range would be between 70 – 120. 70 would likely be when you first wake up in the morning, or haven't eaten in several (5-ish) hours. 120 would be post-prandial (medically speaking, that's 2 hours after you eat).
There is a great documentary being made about a human trial for Type 1 diabetes treatment. This is a comparative chart from their website. https://thehumantrial.com/story/
A1c
Your A1c, or HbA1c (Hemoglobin A1c) is defined by Mayo Clinic as: The A1C test result reflects your average blood sugar level for the past two to three months. Specifically, the A1C test measures what percentage of your hemoglobin — a protein in red blood cells that carries oxygen — is coated with sugar (glycated). The higher your A1C level, the poorer your blood sugar control.
A non-diabetic will rarely go above 5.7% A1c.
What happens outside of normal range? Good question.
Moderately high blood glucose (over 200) over an extended timeframe can lead to: kidney failure, liver disease, retinopathy (deadening of the nerves in your eyes = blindness), neuropathy (deadening of the nerves in your extremities), nerve pain, heart failure.
Extremely high blood glucose (over 300-400) can lead to ketoacidosis within a few hours, leading to coma or death within hours if untreated.
Low blood glucose can happen fast and has much more immediate consequences. When blood sugar dips below 60-70 it can lead to immediate coma, death, seizures, or severe brain damage. The lower your blood sugar dips, the more it effects your brain. Some people have seizures when they hit the mid-50s. I've been as low at 34 and never had a seizure or coma, but man the 24 hours following are sure shitty.