fre·quen·cy (frē′kwən-sē)
1. Physics The number of complete cycles of a wave, such as a radio wave, that occur per second. See more at wave.
2. Mathematics The ratio of the number of occurrences of some event to the number of opportunities for its occurrence.
Source:http://www.thefreedictionary.com/frequency

Its just how often you do a certain action.

Yes exactly. So say you are on the BTN with a somewhat marginal hand and you do not know if you should fold, call or raise MP`s raise. One metric that you could look at is how often villain folds to 3bets (--> what his fold to 3bet frequency is). Say he folds to a 3bet 95% of the time then you could conclude that you should probably 3bet all of your hands against this opponent because his folding frequency is so high that it is a +EV line.

Same would go for a cbet frequency. Let`s take the example from above but you only call. Then villain cbets the flop. In his stats you can see that he has an extremely high cbet frequency (i.e. 100% --> so he cbet 100 out of 100 hands). This might lead you to call his cbet wider than if he only cbet with a frequency of 10% (and never check-raises) for example.

So in conclusion a frequency is the percentage of times you do an action divided by all the times you had the chance to take that action.
Hope that helps.
Good luck at the tables.

sure.

Frequency = (x amount of times a specific action takes place / all instances where that specific action could have taken place) * 100

so for example, let`s say UTG opens and you 3bet him. out of 10 times (all instances where that specific action could happen) he calls you 2 times (amount of times a specific action takes place. This would result to the following equation of villain calling your 3bet:

Frequency = (2/10)100 = (0.2)100 = 20%.

Hope that helps.

thanks