Constants is the name that we have given to the symbols or functions that we use in maths. Symbols like Pi, Squareroot, Power etc which are not just symbols but have a purpose or a value are listed here.
When you need to use any one these, hover your mouse on the button of the constant and a tip for using it will appear. When you click on the button, the symbol is appended to the end of the formula, as it is currently typed. Don't try and cut/paste this symbol to change its position, as this probably might not(or probably will) work.