Meta search engines are search engines that search other search engines. What this means, is that when you enter a keyword query into a Meta search engine, it, in turn, passes the keyword(s) you have entered on to other search engines and initiates a search with these other search engines. It gathers the results from each of these search engines and then collates the results into one single set of results which it then presents to you.
The idea of Meta search engines is that they supposedly undertake a more thorough search and obtain more comprehensive results covering the best of all the search engines. Others would argue, however, that Meta search engines simply complicate matters by generating even more results that any searcher can handle.
Examples of Meta search engines: