The standard chainring is 53-39, indicating the number of teeth on the large and small chainring (click to enlarge).
No one likes math (not us at least). But cycling gear ratios have become a hot topic with the advent of compact cranks, sub-compacts and even 1x drivetrains. In simple terms the number of teeth on your cogs and chainrings are combined to determine how well you can go uphill, downhill, or zip along the flats. And while it's tough to say exactly what ratio (read: gear combination) will be best for you, this video from the Global Cycling Network does a nice job of breaking down the basics. Afterwards, if you want to do some gear ratio comparisons, check out this handy calculator.