Disclaimer: IANAL
"Open Source Software" means that you must give a copy of the sources to the user in some form. So if you include OSS in your product, you need to comply to the rules in the license.
For example, if you include a single GPL library into your code, then your product must be released as GPL software (i.e. with full source, under a GPL license including the permission to change the sources and create derivatives).
Most frameworks have a less strict license. For many OSS licenses, it's enough when you include a copy of the original license, name the library in your documentation in a prominent place (+ URL of the home page) with the name of the license it uses.
For example:
Used Open Source Software in This Product
This product uses the following open source software: