"You have the watches, but we have the time."
The above is a double-entendre, meaning it has two meanings, a superficial meaning and a deeper meaning.
At first blush, or superficially, it looks like by "watches," it's referring to wristwatches, wristwatches almost always being simply called "watches."
When one doesn't have a watch but wants to know what time it is and sees someone who does have a watch, a common way of asking that person what time it is without being presumptuous is to ask, "Do you have the time?"
Based on this, people with watches (i.e., wristwatches) have the time (i.e., have on their wrist what time it is).
Therefore, since people with watches have the time, it's funny to say, "You have the watches, but we have the time." That funniness, that oddity, is how we get cued to this being a double-entendre, clued into the notion that the superficial meaning we would surmise at first blush isn't the deeper meaning that's actually intended. But what is that deeper meaning?
When you know the context, that it's being said by the Taliban in reference to the US military presence in Afghanistan, the other meaning becomes clear:
In the military, armies maintain control and keep an enforced peace by having soldiers keep watch in order to immediately respond to any uprising or disturbance of the peace. A single patrol or incidence of this is called a "watch" (e.g., There's a watch at Kabul's South Gate. Corporal Riley starts his watch at Kabul's at 0600.). In Afghanistan, thousands of soldiers are keeping watches all over the country to maintain the peace and keep the Taliban at bay.
However, the Taliban expects the US can't stay in Afghanistan forever, won't keep all its watches forever. Sun Tzu in the Art of War touches on this, essentially saying that the strategy for a domestic force to take against an invading force that has it outmanned and outgunned shouldn't be to win but should be to simply not lose, thus never giving up and turning it into a waiting game. For the domestic forces, it's their homeland, so they've got nowhere else to go or be, no real reason to give up, but that's not true for invading forces. Those soldiers want to eventually go home. The reasons for invading eventually dry up as the invaders' needs, wants, politics back home, leadership, etc. shift and change over time. So by never giving up and making whatever trouble can be made whenever and wherever possible, even if just a bit to let the invading forces know that they're ready to take the country back the minute their enforced peace through watches lets up, the invasion forces will eventually tire, will eventually reach a point where the cost of staying outweighs the benefits of staying, and while not beaten, will withdraw just from no longer wanting to be there.
With all that in mind, what "You have the watches, but we have the time" actually means is:
The US has the watches (i.e., soldiers keeping watch in thousands of watches all over the country to presently keep the Taliban at bay and without control of Afghanistan), but the Taliban has the time (i.e., the time to wait until the US gets tired of being in Afghanistan and goes home, at which point the Taliban is suggesting here that it will then be unopposed in Afghanistan and immediately seize back control of Afghanistan).