'Opportunity cost' is a relatively simple concept, and one of the few in economics that is not completely trivial but still truly makes sense.
The idea is that the real cost of any choice is the value of the other choices you could have made. That is, if you have 100$ and can buy either a farm or a factory with them, the cost of the farm is not 100$, it is the wealth that you could have gained if you had bought the factory instead (over some time horizon). Of course, in many real-world situations this is not a computable value, as the alternatives are far too open-ended, but still it's a useful way to model certain problems.
The idea is that the real cost of any choice is the value of the other choices you could have made. That is, if you have 100$ and can buy either a farm or a factory with them, the cost of the farm is not 100$, it is the wealth that you could have gained if you had bought the factory instead (over some time horizon). Of course, in many real-world situations this is not a computable value, as the alternatives are far too open-ended, but still it's a useful way to model certain problems.