The term "open source" is one that normally describes a non-proprietary license for software or technology. The idea is that the creator provides his/her source code freely to everyone, so that others may manipulate the program to fit their own needs. This often results in a large community of contributors working to develop improvements and additions to the work.
Open source may apply to software, 3d-models, or really any form of technology where the creator provides the fundamental plans to anyone who wants them. Normally such plans are protected by a version of a Creative Commons license, which generally means it is free for anyone to use provided credit is attributed to the original creator, it is not used for profit, and any derivations from the original design are also provided as open source.