Quick meaning
Software whose code is freely available for anyone to view, use, modify, and distribute.
Think of it like this
Like a recipe that anyone can read, cook, and improve upon.
Where you may see this
You may see this inside app stores, app settings, software updates, and install instructions.
Why it matters
Knowing the term helps you understand what an app is asking you to do before you tap, install, or change anything.
Say it simply
Open Source means: Software whose code is freely available for anyone to view, use, modify, and distribute.