The engine has different licenses including:

For Open Source Projects:
If you are developing and distributing open source applications under the GPL License, then you are free to use djondb under the GPL License.

For Commercial OEMs, ISVs and VARs:
For OEMs, ISVs, and VARs who distribute djondb with their products, and do not license and distribute their source code under the GPL, djondb provides a flexible OEM Commercial License.

For Web Sites, Enterprise IT, and Government IT:
djodb Enterprise is the comprehensive subscription offering that provides database developers and DBAs with everything they need to successfully develop and deploy database solutions with djodb. 
It includes Certified Software, updates and upgrades, proactive alerts and advisors, the online djondb Knowledge Base, and full production-level technical support. 
The Certified Software (database server, connectors) is provided under the GPL License. Optionally, customers may choose a Commercial License.

The licenses are included here for further reading.

All the files included are licensed under GPL unless the file explicit express some other license.

Please refer to COPYING.GPL and COPYING.LGPL to get a copy of the terms of these licenses.