Open source
Open source means the code is public, so anyone can read it, use it and often improve it.
In a simple example
A free calendar piece that anyone can download and look inside is open source.
Why it matters to you
Many of the tools you use are open source, so it matters to read their licence before you rely on them.
Do not confuse with
Open source does not always mean you may do anything. Each project has a licence with rules.