A bachelor degree is basically the starting point in your university career. Everybody needs one before they can attain any higher tertiary qualifications. A masters is something you attain after your bachelor's degree because it's at a higher level. A good example would be the School Certificate and the HSC. You need to do your School Certificate before you can attempt the HSC, in the same way you need to do a bachelor's first.
My analogy is probably outdated since they've scrapped the school Certificate (have they yet?), but I'm sure you get my drift.