Take a look at the dictionary :

int Highest( string symbol, int timeframe, int type, int count=WHOLE_ARRAY, int start=0)
Returns the shift of the maximum value over a specific number of periods depending on type.

Do not forget that this function return the index of the bar, not a value, so look at the sample :

Sample:
double val;
// calculating the highest value in the range from 5 element to 25 element
// indicator charts symbol and indicator charts time frame
val=High[Highest(NULL,0,MODE_HIGH,20,4)];