Lots of recipes mention the word 'simmer' but don't panic if you do not know what this means.  It is a simple term to understand and do. 

When your pan of water and food is on the hob simmering involves turning the heat to its lowest setting.  There should only be small bubbles on the surface of the liquid.  This means something is getting near to boiling but isn't quite boiling. You should keep it at this level for as long as the recipe says to do so. Simmering is a way of cooking the food slowly and gently.