Highlight row on drop down selection

So say, column A has your drop down lists... and you want to hightlight rows from column A:Z


First select entire range, A1:Z100 (assuming row 100 is last row)

Invoke the Conditional Formatting

Choose Formula Is from 1st drop down and enter formula =$A1="High"

where A1 is top-left most cell in your selection (note the $ sign.. this is important as it maintains reference to column A throughout the sheet).

Choose your format

If you want more conditions... click Add and continue with similar formulas...