ok, here ya go
245 - width in millimeters of the tread itself, not actually the tire
45 - aspect ratio of the tire. So this tire is 45% of 245mm in height
17 - diameter of the wheel your mounting the tire to.
Here is some trick math for ya.
245-45-17 is the same height as a 255-40-17 or a 245-35-18.
When staying with the same rim, when you increase width by 10, decrease your aspect ratio by 5. When changing rims, also known as plus sizing, for every increase in rim size, you want to decrease your aspect ratio by 10.
This will keep the overall diameter roughly the same.