Open source typically means that the code is public and comes with extensive freedoms to use, modify, and distribute (the degree to which these are allowed is governed by the software license).

Source available, on the other hand, generally means that the code is publicly available for review but is otherwise proprietary and/or restricts the freedoms that an open source project provides.

The differences are more nuanced than the above summary might suggest, as they come from different philosophies on what open source should mean and how people should be able to interact with and use open source projects.