Reading Exposure Histogram

[iad.spotter]

Hi all,
I just got three photos screened and all three were rejected for being underexposed. I can see how two were underexposed, but the third I thought looked fine. I'm still very new to photography, so I was wondering how I can read the histogram to not make this mistake in the future. Thanks.

Ben

[TomEPKK]

Links to rejected pictures would help here...