It's just that normal gravity on earth feels exactly like being in an accelerating elevator in space. So you can't tell the difference from the inside. Like in the elevator you can ask them, whether you're still on earth or accelerating in space. Einstein used this thought experiment to develop the general theory of relativity.

Basically Einstein thinking about that weird feeling you get in your gut when an elevator starts upwards led to him concluding that mass bends spacetime making light from distant stars go in curves around the sun, which was confirmed during the next available solar eclipse.