A1C
A1C is the proportion of your haemoglobin that has become glycated, reported as a percentage, and it reflects average glucose exposure over roughly the previous two to three months. It is a weighted average rather than an even one, because the most recent weeks contribute more than the earliest.
Example
An A1C of 7.0% corresponds to an estimated average glucose of 154 mg/dL. What it cannot show is variability: someone steady at 154 and someone swinging between 80 and 250 can report the same A1C.