To expand on this a bit, when you normally delete a file from the trash, the system just removes the information about where the data in the file is located on the disk, and adds those locations to the free list, which is used to figure out where to put data for new files. The data that was in the deleted file is still on the disk until it gets overwritten. It is possible for someone to search through the disk as a raw device, which is basically just a series of bytes that make up all of the files and all of the free space, looking for interesting data. For a file of any significant length, the data from the file will be in several different spots on the disk, so it would be hard to automatically recreate the file.
What secure delete does it write over the data in the file, with zeros or random data, before deleting it. That way, none of the information in the file remains on the disk. This takes a lot longer, as every byte of the file has to be overwritten, instead of just removing some housekeeping information.