A slightly stiff front door lock isn't necessarily a cause for alarm, although it could perhaps become one if the underlying cause isn't resolved. You don't want to be in a position where the lock gives out altogether and you find yourself having to call a mobile locksmith to let you back into your home in the wee hours of the morning! There are many reasons why a front door lock becomes stiff and difficult to operate, although many of these reasons can be rather easily remedied.
This is perhaps the most straightforward solution. Apply a small amount of appropriate household lubricant to the lock and then insert your key to distribute the product. Wait a few moments and try the lock again. Be wary about using liquid lubricants if the lock appears to be excessively dirty. There is the chance that the lubricant will combine with the dirt and dust inside to create a viscous mixture that can actually gum up the lock's internal mechanisms. In this instance, use a graphite powder (which can be found at most hardware shops). Apply a small amount of the product to your key before inserting it into the lock. Jiggle it slightly to distribute the graphite and then try to turn the lock.
Perhaps it's your actual key that has become the issue. The cuts on the key can become worn over time, meaning that it might be on the verge of no longer matching the lock. Don't attempt to redefine the cuts with a file or similar implement so that they match once again as this is tricky to get right. It's much better to just visit a locksmith and have a new key cut.
What about when both your key and the lock both seem to be in perfect working order? It might be that the problem can be found in the door itself. Over time, doors can become slightly warped, causing a misalignment when the bolt attempts to feed into the strike plate (the corresponding hole in the door frame). The stiffness is simply excessive friction from when these components no longer work as well together as they should. It might be that the door's hinges require some minor tweaking to bring the door back into the correct alignment. In extreme cases, door replacement might be needed.
So while a stiff door lock isn't always a big deal, it's important to fix the problem sooner rather than later—when the problem might become far more serious indeed.