Total porosity counts every pore, including isolated ones that fluid cannot leave. Effective porosity counts the connected pores that fluid can move through, and that is the porosity used for flow and, usually, for oil and gas in place.
A porosity of 0.2 means a fifth of the rock is void. The number says nothing by itself about whether those pores are large enough to flow: a chalk can be porous and still nearly impermeable. Porosity is a fraction or a percent.