bootable or not bootable what's the difference?

[QiartMedia]

Hi,
Sorry I maybe having a stupid moment hear but I don't understand the difference between step 2 and  "if you prefer to skip the firmware update step and prepare all your cards from the PC, you may copy ML files on your card and make it bootable with a special utility"
what's the difference of having a card bootable or not?
Gaz

[a1ex]

not bootable = ML does not run from that card