Quick meaning
The size of text as displayed on screen, measured in points or pixels.
Think of it like this
Like choosing large-print or standard text in a book.
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
Font Size means: The size of text as displayed on screen, measured in points or pixels.