In flutter, everything is a widget (yeah we crazy like that 🔥). Widgets are the building blocks of a flutter app- I mean, that's what every article says. Below are some articles that will introduce you to widgets.
To understand what widgets are all about in flutter, using a burger analogy :
A guideline to widgets you need to get familiar with before building your first flutter app, these are basic widgets:
A reference list to all widgets in Flutter:
You can also check out Flutter's official Youtube channel for 'Widget of the week' series, these are short videos explaining different widgets in Flutter
And then for more videos: