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I have a acer aspire 5610 and on the left side of the laptop i have a slot for something - i dont know what its for. its on the corner ont he left closest to the user. it also has a button that can be pushed out and left out but also clicked in so it stays in.the bit that comes out is a rectangle piece of plastic with 6 squares inside it, thin also.

You will probably have an inbuilt wireless intel connection chip. but some crappy routers wont accept that intel chip - or sometimes its for users that have a desire of keeping more security over their wireless internet that thing your talking about is a plastic filament just to make sure nothing gets in it whilst there is no wireless connection card in it. main answer to you question is - its for a wireless card that comes with many routers.