In production and development, open source as a development model promotes a universal access via a free license to a product's design or blueprint, and universal redistribution of that design or blueprint, including subsequent improvements to it by anyone
To put it a bit more simply (or at least less succinctly) than debastian did (which was absolutely correct) is that open source software (and hardware) is not proprietary and can be viewed or modified by anyone without cost. That doesn't mean it is not protected by copyright, but the terms of those copyrights allow you to access the code, modify it, and redistribute your modifications, though you will likely have to make your changes available to those you redistribute it to. All of the GNU tools, Apache tools, and many others are open source software. Open source hardware is a newer development and I can't point to many examples. Arduino and Raspberry PI stuff may be.
In general, open source refers to any program whose source code is made available for use or modification as users or other developers see fit. Open source software is usually developed as a public collaboration and made freely available.
A simplest relpay to this thread is that open source denoting software for which the original source code is made freely available and may be redistributed and modified
open source does not mean no cost. There can certainly be a cost involved, but generally that cost will be no more than the cost of delivering the code to you (e.g. a company I used to work for had an open source project, the source would however be distributed only on CD-ROM on request, at a cost of something like $25 shipping and handling (postage, administration, cost of the media).