Those numbers are pressure measurements. Standard atmospheric pressure at sea level (ATM) is equal to:
1 atm = 29.92 in Hg (inches of mercury)
1 atm = 760 mm Hg (millimeters of mercury)
1 atm = 1013.20 millibars
1 atm = 14.7 psi (pounds force per square inch)
1 atm = 1013.20 hPa (hectopascals)

Readings above or below would be high or low pressure.
High pressure = clear weather
Low pressure = clouds/rain
Rising pressure = clearing
Falling pressure = clouds/rain

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Hi Mike, the above link will what the weather is doing at any given point of the day.

Mainly, the gray, sometimes circular lines are pressure gradients that move across the US with certain weather patterns. High to low is the key, you can derive which way winds usually turn inwards towards the low (above the equator) and away from the highs. When you have the circles stacked, and the charts are indicating movement, you have a change, sometimes drastic coming.

Lots of really cool weather stuff available from that link that you can correlate to your app.