Treadmill heart monitoring question?

Hi there, i recently started using a treadmill with pulse sensory pads on the handrails. When running my heart-rate gets to around 90-100bpm which i presumed was normal. However, on the treadmill it has a little chart which is titled 'heart rate training zones' and for my age it says my heart rate should be between 125 and 165bpm. Obviously i know that if i run faster i will be able to achieve this but i just wanted to know what it means. I run pretty fast to get to around 100bpm so what do these 'training zones' actually represent? Thanks for helping if you can understand the question!