The screws mate into 6 threaded holes on the core. If the plastic threads inside the holes fail, it prevents the screw from tightening and it may let go of the screw entirely. When this happens the threads are said to be "stripped".
It can be caused from overtightening of the screws. If you tighten a screw until the spring is fully compressed, then any additional tightening will put excess force on the plastic threads. The screw itself will actually rip out the plastic threads if you continue to turn it. Other causes can be rough handling of the cube or core plastic that is too soft and easily damaged.
If the hole is large enough, one possible fix is to get a little piece of paper down into the hole. Then you can get the threads of the screw to bite into the paper which may be strong enough to hold it. If it were me, I'd just order a replacement core from an online cube shop. They typically only cost about a dollar.