Forces aren't getting weaker, exactly. They're just made to act across a larger gap.

One way to thing about it, is to imagine that every point in space "splits" into multiple points all around it (if space is a 3D grid, then every point "splits" into 3X3X3 points centered around the original point). This of course is wrong since space isn't a "thing" that can split, but it's a way to help our limited brains internalize the concept and develop an intuition around it.

Another way to think about it, is to consider force carriers. The photon, for instance, is the carrier for the electromagnetic force. The gluon is the carrier for the strong force, etc. If you're comfortable with the notion of force carriers, you can imagine these carriers are "slowing down". This is a bit more wrong than the previous analogy, because an expanding space mean that not only do "forces take longer to reach" from one particle to another, it also means the force a particle exerts on another is weaker. The analogy, however, helps give a new perspective on the concept of expanding space, so it's worth playing with.

It's important to note that the expanding space isn't really measurable on small scales yet, and won't be for MANY MANY years. The rate at which space is added between two points is proportional to the distance between those two points (again, consider the "splitting space" analogy, the more space, the more "splits" per unit of time), and anything smaller than intergalactic scales is so slow, that the standard-model forces (strong, weak, EM and gravity) all counteract it completely. Only in the vast nothingness of intergalactic space is there enough space and little-enough of anything else for this expansion to be the dominating affect. For now... (if the expansion of space is indeed accelerating)