According to Molinists, people have libertarian freedom, and God knows what each person would freely choose to do in whatever circumstances they might be in. That means there are counter-factuals to human freedom. A counter-factual is an if/then proposition. For example, if Pete were holding a drink at such and such moment, then he would drink it.
Since, according to Molinists, there are counter-factuals of human freedom that tell God exactly what each person will do in whatever situation, that means there are possible worlds that God could not actualize even if he wanted to. That follows from the counter-factuals.
Let's imagine two possible worlds that are exactly alike up until some precise moment that we'll call t. From t onward, the two worlds diverge because although Pete is in exactly the same situation in both worlds, he makes a different choice in each. In one world, Pete chooses to drink at t, and in the other world, Pete chooses not to drink at t.
Suppose God knew before creating anything that if Pete were holding a drink at t, then Pete would freely choose to drink at t. With that being the case, God could not have actualized the other world in which Pete were in the exact same situation at t, but chose not to drink. Even though the other world describes a possible state of affairs, God could not have actualized that world. That's what Molinists mean by "infeasible." An infeasible world is a possible world that God could not actualize because it describes a possible state of affairs that contradicts one of the counter-factuals that God knew before creating anything. If it was true all along that if Pete were in such and such situation at t, then he would freely drink, then God could not have actualized the world in which Pete was in the exact same situation at t but chose not to drink.
It makes you wonder how many possible worlds are infeasible compared to the feasible worlds. You'd think there would be at least as many infeasible worlds as there are feasible worlds since for whatever choice a person makes, they could have done otherwise, given libertarian freedom. But there are Frankfurt cases that show libertarian freedom does not require that a person could have done otherwise even if they have libertarian freedom. Still, it seems that in most of our choices, we could have done otherwise if those choices were free in the libertarian sense.
I think there would actually have to be far more infeasible worlds than feasible worlds. After all, at any moment, it's not as if the very next choice you make is either to act in some particular way or not to act in that way. At each moment, there are a whole slew of things you could choose to do. Let's say you decide to point in some direction. Well, there are a whole bunch of directions you could have chosen to point. You could choose to point north, south, east, west, up, down, or anywhere in between. You'd have 360 degrees to choose from, and that's just in the horizontal plane. But if you choose to point at some precise moment, you're only going to choose to point in one particular direction. There is some counterfactual that says exactly which direction you're going to point if you are in that situation at that particular moment. Yet every direction is possible. For every possible direction you could point at that moment, but don't, there is a possible world in which you do point in that direction, but each of those possible worlds are infeasible for God to actualize.
Think about that. At any given moment, there are a whole slew of things we could freely choose to do, but we don't. We only take one course of action. That means there must be far more infeasible worlds than feasible worlds. Of all the possible worlds, there were relatively few that were feasible for God to create.