From the picture above, it appears that the bearing at the bit end of the motor is exposed. The primary (thicker) nut against the bearing has two purposes, 1 it maintains the preload on the bearing, and 2 it has at least one vertical slot in it that is used in conjunction with the spindle lock lever to lock the spindle for changing bits. The thinner lock nut above the primary nut is fastened tightly against the primary nut effectively locking both to the spindle.
The nuts would be set at the factory and should not need to be touched again unless there is a need to completely dismantle the router. Neither should be touched with a wrench while changing bits because of the risk of varying the preloading of the bearing or allowing vertical play to be established in the spindle, that bearing is all that can lock the spindle protrusion in the motor assembly, misadjustment either fries the bearing or allows the spindle to float in the motor housing giving random variation in bit depth.
For changing bits, unplug the router, engage the spindle lock, use a wrench to loosen the collet nut, remove and replace the bit, tighten the collet nut with the spindle lock engaged, disengage the spindle lock and set your cutting depth.