A bootloader is a small program which is started by the computer at boot time. It's responsible for starting up the whole system. Normally, the bootloader installation is totally automated. DrakX will analyze the disk boot sector and act according to what it finds there:
If a Windows® boot sector is found, it will replace it with a GRUB/LILO boot sector. This way you will be able to load either GNU/Linux or any other OS installed on your machine.
In any other case it will ask you where it should place the boot loader. Generally, the First sector of the drive (MBR) is the safest place.
Choosing Skip won't install a bootloader. Use this option only if you know what you're doing.