Yes, this is an old fashioned Angel Food Cake "quick release" pan.
You turn the cake over in the pan, so it's upside down, and you slide the tab away, which is supposed to make it easier for the cake to be released.
If the tab is stuck, then it may just be from age and never being used. I would not force this, as it can be broken.
If you have a pot big enough to stick this in, to have it in boiliing water, then you can try it.
If it's got years of cake batter baked into it, or it's stuck because of other kinds of buildup, it could be difficult to get free.
If the boiling doesn't work, then you might try soaking the tab and seam with vegetable oil, coating it really well for about three days. Or, if you want, lay that part of the pan down inside another pan or container and then fill it up with the oil until the entire tab/seam is covered and leave it for a few days. Wash it off with hot soap and water, and then try to remove it.
If you can remove it, then try and use an old toothbrush to scrub away the gunk.
If not, then you might just leave it as is. There some OTC acids out there for aluminum that can dissolve the gunk thats built up, but I don't know what it will do to the metal.