URL is an acronym used in computing and in academic writing, particularly in discussing academic references. It stands for the words Uniform Resource Locator - the address on the internet that will only find the material for which you are looking. This is the address that you type into your search pane to find a particular source. It will often start with the letters "http://www.", and finish with ".org", ".com" or for academics in Britain, ".ac.uk". (Universities in the USA have the termination ".edu".)
Serious geeks in computer circles will tell you not to confuse the URL with the URI - the Uniform Resource Identifier. To such technical experts, URLs form only a subset of URIs. Most of the world can ignore such pedantry.