RPM means the head rotation per minute.
This is the speed of the HDD… the faster the head spins the faster the HDD can write and read the data from the disk.
Usually the 7200rpm is a very good value so if you look for a HDD then I recommend purchasing the HDD supporting such speed… but this value is not very important for the whole notebook performance… is good to have but not very necessary…. You know…
Higher rpm (revolutions per minute) is better or let’s say faster. Data can be read a little bit faster but higher rpm causes more heat, noise and higher power consumption.
Lower rpm is simply the opposite.
In my opinion it’s depending on the applications that you use. If you use the notebook just for Internet and e-mails a HDD with 5400 rpm is enough. If you often do office work, picture editing, etc. I would take a model with 7200 rpm.