A bow's AXLE-TO-AXLE length is just what it sounds like. At the end of each limb is a steel axle which holds the rotating eccentrics (cam or idler wheel). The distance between the centers of those two metal rods (axles) is the bow's advertised axle-to-axle length.
Of course, the bow's actual physical length (measured height if stood on its end) is always considerably more than the axle-to-axle length - since the eccentrics extend beyond the axles.
The vast majority of today's hunting bows have axle-to-axle lengths of approximately 32". Bowhunters clearly prefer shorter bows in today's market, and 32 seems to be the magic number. Bows with axle-to-axle lengths around 34" are often considered "crossover" bows - suitable for hunting and competition use. Bows with axle-to-axle lengths of 36" or more are primarily considered competition bows.
The market has some tolerance for even shorter bows - down to 30" axle-to-axle (commonly sold as "treestand" bows) but success for sub-30" bows has been sporadic. Bows with axle-to-axle lengths in the 25-29" range are typically youth-only models.