There are a couple of situations that can be described as requiring proper timing in an action. The most common is when, in the the bolt's cycle, the cocking piece engages the trigger. If it is too early, there is cock on closing (Some of the work of cocking is done at the end of the forward movement of the bolt, increasing the effort required to operate it.), and more effort required to cycle the bolt, and if it is too late, striker fall is decreased, reducing the amount of energy with which the primer is struck, and impact on the trigger mechanism is increased.
Adjustments to this aspect of action timing may require adjustment of the cocking piece's position on the firing pin (in cases where there is such a provision, usually by a threaded connection), or modification of the cocking piece, or in the case of actions that have them, adjustment of, or switching the trigger hanger for one that positions the trigger in a more satisfactory location within the action.
Another relationship that can require attention is the location of the bolt handle relative to the locking lugs. If the handle is not properly located, the lugs may still be engaging flat portion of the lug abutments at the point that the primary extraction cam engages, causing binding, or if too far the other way, the lugs will not be fully onto their abutments when the bolt is at the bottom of its slot in the action body.
There is also the matter of the handle's position fore and aft, on the bolt, which if incorrect can cause it to hit the extraction cam too late in the bolts rotation, reducing or eliminating the length of pull that the cam produces during bolt operation. The other thing that can be affected by the position of the bolt when closed is the position of the cocking piece within the cocking cam notch. If the bolt closes too far, the cocking piece can rub on the side of the notch opposite the cam.
Getting an actions timing right makes its operation smoother, as well as guaranteeing that the amount of striker fall is correct, and a maximum of extraction travel is available. Pretty much all of this work is better off left to a qualified gunsmith, who has a good understanding of these issues. I am not sure that all do.
