Mawdu': placed to carry meaning
مَوْضُوعmawdu' - assigned meaning
Mawdu' means a word or expression placed to carry recognised meaning, the doorway into Arabic grammar.
Simple explanation
مَوْضُوع (mawdu') is an uttered expression that has been placed to carry a recognised meaning.
This is where grammar begins, because nahw studies meaningful words and how they come together to form phrases and sentences.