Plantinga's ontological argument, part 1
Before explaining this argument, lemme explain possible world semantics first. Possible world semantics is just a tool for talking about and thinking about possibility and necessity. Possibility and necessity are taken in the broadly logical sense. That means when we say, "X is necessary," that means it's impossible for X to be otherwise. If we say, "X is possible," that means there is nothing logically impossible about X.
For example, married bachelors are impossible because it entails a contradiction. On the other hand, it's possible that the Battle of Little Bighorn never happened. Now it isn't actual that it never happened. It's just that it could have been otherwise. There's nothing logically incoherent about it not happening.
Instead of talking about necessity and possibility, we can talk about possible worlds. A world is a maximal state of affairs. Take the actual world for example. The actual world is the sum total of reality. If you took every true proposition about reality, they would describe the actual world. Let's say we hypothetically change one thing. Instead of wearing a white shirt today, I might have worn a blue shirt. Instead of saying, "It's possible that I could've worn a blue shirt," we can say, "There is a possible world in which I'm wearing a blue shirt." For every little possibility that exists, there exists a possible world in which that thing is true. So possible world semantics is just semantics. It's just a way of talking about possibility.
As you can imagine, there's practically an endless number of possible worlds. For everything that might be or could have been, there's a corresponding possible world.
If there is some proposition that isn't true in any possible world, then that means it's impossible for it to be true. For example, "There's a married bachelor walking down the street," isn't true in any possible world since it's logically impossible for there to be any married bachelors.
If something is true in all possible worlds, then that means it's necessary. It's impossible for it not to be true. All tautologies, for exampmle, are necessary. "Circles are round," is true in all possible worlds. (Equivocation isn't allowed, so you can't just start redefining words.) The laws of logic are also true in all possible worlds. "Truth exists" is true in all possible worlds, because the denial of it is logically impossible since it's self-refuting.
If something is true in some possible worlds but not others, that means it's possible. A think needs to be true in at least one possible world in order to be possible.
Let that soak in for a minute and I'll explain Plantinga's argument later. If what I said isn't clear, then just search for "possible world semantics" on the internet and read up on it.