Short answer: A magic number (0xAA55) being at the end of the first sector on the floppy.
Slightly longer answer: "Booting" means that the first 512 bytes of the floppy are copied to memory (usually at real mode segment 0x0000:0x7C00 or physical address 0x7C00). This is the first sector of the floppy, sector 0. It's also known as the bootsector. The processor then jumps here and starts execution. Some older BIOSes will check that the last 2 bytes of the segment are 0xAA55. This is a magic number.
DD is a tool that is used to write to specific sectors on the disk. It is one of the more preferable tools for accessing raw disks on *nix systems.
EDIT: This is for IBM-PC & compatibles. See this article.