The term open can be used to described a variety of different techniques and relationships in technology industries. "Open" systems generally refer to the ability of third party other than the original creator/owner to create additional or compatible materials. For example, an open hardware design might mean a third party can create a product, which plugs in and works with the original design. Open usually, but does not always, mean free (i.e. without charge) and freely available (i.e. relevant information available to all).

Computer software can be protected under various intellectual property laws.  See the discussion of intellectual property laws applicable to software elsewhere on this site.

Open source is a kind of software licensing in which both source code and object code is made available to licensees. Open source is intended to accelerate and "democratize" the software development process. In contrast to open source, commercial software developers would normally only make object code available to their customers, and jealously guard the source code.

This different approach is important because of the important difference between object code and source code. Object code is the form of software which is directly executable by a computer, but which cannot be easily modified to create improved or competitive programs. Source code, in contrast, can be easily understood and modified by competent software professionals. Also, trade secrets can be hidden in object code. They are exposed in source code. Once exposed, trade secrets are no longer secret, and cannot be protected under trade secret law.

Contrast with Public Domain. Open source is not the same as public domain materials. Also, open source can actually describe quite different types of licensing procedures.

If material is in the public domain, it means it is entirely unprotected by copyright, trade secret or other intellectual property law. An example of public domain materials would be books for which the copyright has expired, or patented processes for which the patent has expired. Any person can take it and incorporate it into a new product without any obligation to the creator or any third party. In fact, they can incorporate it (in whole or in part) into new products which they sell as proprietary systems

In contrast, open source software is protected by copyright law, and is licensed. In part this is to accomplish the goal of keeping further development of the materials open. Generally, open source licensing arrangements provide mandatory access to source code for "downstream" customers. If you use the open source materials in your product, you must provide your customers the source code at no (or nominal) extra cost.

Why Open Source? Open source has advantages and disadvantages. For a discussion, look on this website at Why Open Source.