So, presuming you know broadly what a constitution is (ie. the rules and systems for governance of that country - obviously very rough definition) then its pretty simple. A constitution is a generic term, so for example 'constitutions around the world differ from each other'. A Constitution is specific to a country, for example the Constitution of the United States. More often than not, Capital-C Constitution is used for countries which have a written constitution such as the US, and small-c is used for countries which don't have a formal Constitutional document etc such as the UK.
Google Anthony King, he uses this distinction and reading his work will help clarify this further.