Open Source Software is something which you can modify as per your needs, share with others without any licensing violation burden.
When we say Open Source, source code of software is available publicly with Open Source licenses like GNU (GPL) which allows you to edit source code and distribute it. Read these licenses and you will realize that these licenses are created to help us.
Motive behind making these software is to create something which will be useful to users. When we say Open Source is free, the word free is not attached with cost factor. Open source software can be paid but source code must be attached to it. Also one of the main income source for these softwares is service. By creating such softwares, you can always charge for services.
Simple philosophy of OSS is when we buy vehicle then one can modify it, change its parts without manufacturers consent (at cost of warranty of product though)but in software world this was not possible due to strict guidelines of EULAs we generally don’t read. OSS brings concept which helps us to use software same as we buy other things.
Software for which the original source code is made freely available and may be redistributed and modified according to the requirement of the user, this is also free to download.
Open Source celebrates the free availability of source code and distribution. Thus, open source software allows for computer programmers and developers to “stand on the shoulders of others” and create their own software.