The trash can is just a temporary place where you delete things to, in case you press delete accidentally then decide you don't want it deleted.
Open the recycle bin, and go file->empty recycle bin, in order to properly empty it and have more free space on your computer.
As Mac70 points out though, due to the technical way that computers work, even now it isn't totally destroyed, so you can use a program like eraser to truly destroy and erase something, if it's very sensitive information like your bank details or something.