Hertz (Hz) is a unit of frequency or "how many times something occurs per second". For most things, this refers to periodic "cycles", so it's referred to as "cycles/second".

With RAM, the allows can be given in clock frequency MHz, which means "millions of clock cycles per second" or it can be given as data rate.

In the case of the latter, the unit is technically also MHz - aka "millions of data transfers per second" - but because that unit is confusing when shared by the related metric of clock speed, the industry is adopting MT/s or "Mega-Transfers per Second" to represent data rate.

DDR or Dual Data Rate means that the data rate is twice the clock speed, so marketing has long advertised RAM speed using their data rates as it is a larger number. They'll often, somewhat misleadingly (but still technically correctly) use the unit MHz for this data rate, instead of MT/s as noted above.

This means that when your computer displays the clock speed in MHz, it's a separate value from the MHz that was on the box of your RAM, since that was the data rate.