Blockly is the block code editor used in App Inventor. Blockly provides a way to write programs by arranging blocks of code, rather than writing out traditional “source code” text programs.
This approach is helpful for new programmers who can focus on the programming – and not the details of the structure of the programming language (of which there are many in programming languages like Java, C/C++, and C#).
New programmers are fighting two battles at once: the syntax battle and the logic battle. Blockly eliminates the syntax battle, it is impossible to make a syntax error in Blockly. Students can focus entirely on the logic behind conditionals, loops, variables and other core concepts without worrying about unbalanced parenthesis or missing semicolons.
There’s more about the history of Blockly at the linked article.
And there is this funny: