To put it simply, it is in fact a software whose code is designed to be publicly accessible.
The open source is actually a license applied to software, but also to any intellectual creation.
In the scenario of a software, this license thus indicates that everyone can check its source code.
Everyone will then be able to, if it wishes, propose to the community its modified version in order to, among other things improve it or solve bugs within the program.
A large number of people can suggest updates, which generally makes the code more reliable.
Be careful though, open source is not synonymous with free of charge, even if some software is distributed for free.