As @Dee says, if you mean the white plastic slider, you can remove it even if stuck. If it is in the upper slot, the bees often put propolis on it to seal up the gaps. Here is a previous discussion of what to do. I have tried it, and it works:
As @Dee says, if you mean the white plastic slider, you can remove it even if stuck. If it is in the upper slot, the bees often put propolis on it to seal up the gaps. Here is a previous discussion of what to do. I have tried it, and it works: