Frequency here refers the the frequency of AC electricity and typically these are either 50Hz (UK Mains), 60Hz (US Mains) and 400Hz (used by many aircraft).
The term Hz means Hertz and is named after Heinrich Hertz and it is the designated SI unit for frequency. The ferequency is the number of cycles per second.
The main issues which are presented by the different frequencies required either by different countries or industries is the fact that certain types of electrical equipment either will not work or will behave in a strange way if presented by the wrong frequency. If for example you have a fridge which is designed to use the US system of 60Hz it may not work correcly in the UK.
When choosing a DC Power Supply (including things such as Laptop and Phone Chargers) the frequency of the AC mains is not normally an issue as the rectifier circuit will often be built to accept a wide range of frequencies.
SImply power supplies can also provide frequency converters
which are capable of accepting one frequency and converting it to another - if you require this type of unit then please get in touch.