im assuming you misread the 2.4 mhz and it really means 2.4GHz. otherwise your computer is 30-40 years old and wont have 1333mhz anything.
but i digress.
hertz or the "Hz" in MHz is how many cycles per second. in computing a cycle is going from having a charge to not having a charge or vise versa.
so with kilo meaning 1000, at 1 Kilohertz (1KHz) the device goes through 1000 cycles per second.
now megahertz (MHz) is million hertz and gigahertz (GHz) is billion hertz.
your average stock destop computer now a days has a processor running between 2GHz and 4GHz. people who overclock get higher speeds the record right now is at 8GHz and some change. but that took some crazy cooling to stop it from burning up.
the 1333MHz you saw is most likely your ram, as 1066,1333,1600,1866 are the most common speeds of ram being sold today. RAM stands for Random access memory. its where your programs are stored temporarily while your running them. we do this because RAM is much faster than your Hard drive. making your program more responsive.
this is why some people suggest getting more ram to speed up your computer. for most people 4 Gigabytes (4GB) is enough. but 8GB is so cheap i recommend that now, as its hard to use up 8GB.
for average computing, ram speeds are not that important. you wont be able to sit down at a computer and just from running programs be able to say if its fast ram or not.