The RuneScape skill calculators are a tool that display the number of times you need to do a certain action to a target level in a given skill, based on your current XP. The XP can be entered manually or retrieved automatically from the hiscore. You will also see your progress and the XP you need to gain the target level.
You can open the calculators as separate pop-ups, which will not display the header and menus. Pop-ups may be useful if you want to have the calculator open next to your RuneScape window.