The definition according to the Open Software Initiative is "Software where the source code (the language in which the program is written) is freely distributed with the right to modify the code, and on the condition redistribution is not restricted, and indeed is obtainable for no more than the reasonable cost of reproduction." As opposed to proprietary software that provides you with executable binary code only, not the human readable source code it is derived from.
Another variation of the definition by The Free Software Foundation (FSF) refers to "free software" rather than Open Source Software. The FSF's definition stresses "free software is a matter of liberty, not price.' In this context, "free" means "freedom" as in free speech, not free as in "free beer'. Some might be asking, "What's the catch"? And you remember what your mom told you, if it's too good to be true, it must not be true. But in this case, it's true, if you find a software product that meets your needs, there isn't a catch, it really is FREE. The only possibly issue is misunderstanding the term "free." The GPL includes this text: "When we speak of free software, we are referring to freedom, not price." which means the software is not necessarily cost-free and considering the alternatives of proprietary software, it's still a bargain.