Wikipedia defines Open-source software as "a type of computer software in which source code is released under a license in which the copyright holder grants users the rights to study, change, and distribute the software to anyone and for any purpose." In openIMIS's case, the copyright holder is the Swiss Agency for Development and Cooperation (SDC) and they allow anyone to redistribute it and/or modify it under the terms of the GNU AGPL v3 License.
In addition to the license agreement, and the ability to modify and redistribute it freely, open-source software is a collaborative effort among a community of users, implementers, and software developers. These communities share their experiences, discuss new features, functionalities for the software, and ultimately provide direction to the future development of the software. You can find more information about the openIMIS community here.