Open source licences can be confusing, but not more than commercial licences.
There a few guideline you need to be aware of, without the legalese, that will hold for most, in no order.
Don't deny others the rights that you had. For example, you had access to the source code, you need to tell others where they can access the source code from.
You don't own the code that others wrote, so don't pretend to. You might be able to own the additional code, enhancements and fixes, but in general, the code you got from the public domain will always belong there.
With most licences, if you distribute the software (for example for download), you have to make the changes available under an acceptable licence. This may differ from licence to license.