Porous simply means containing pores, or holes. In other words, anything that has pores is termed as porous. For example, filters have pores and are hence porous. Similarly, soil is porous and so is our skin (that is what enables sweating in warm weather). 

There are a large number of porous surfaces in nature. We also engineer a number of surfaces to be porous. As already stated, soil and skin are porous; similarly, we design filters for various applications to be porous (with different porosities). For example, microbiologists often use a filter of 0.25 micron or 0.1 micron size to filter water and collect microbes for further examination. Groundwater hydrologists study subsurface porosity to learn more about the transport of water and contaminants through the subsurface. At home, we use materials of different porosities for filtering various items (such as filtering tea or used oil).

Hope this helps.