Doodling
Scribbling, Graffiti
noun, verbal noun
When you fall into the urge to write, you just grab whatever's nearby and start doodling on it.
落書き means doodling or scribbling without much thought, and it also means the scribbles themselves. Imagine a kid doodling in the margins of their notebook in class. Graffiti can be 落書き too, but usually this refers to vandalistic graffiti rather than the artistic stuff — think of an "anarchy" symbol under a bridge or a stick figure drawing in a bathroom stall.
This is a jukugo word, which usually means on'yomi readings from the kanji. 書, however, is kun'yomi in this case and also has rendaku. So just remember that it's read as が for this vocab and you'll be fine.
ここに落書きをしたのは誰だ？
Who did the graffiti here?
いやいや、ただの落書きですよ。
No, no, no, it's just a doodle.
新宿駅の地下道には、昔、たくさんの落書きがしてありましたが、今はほとんど見当たりません。
There used to be lot of graffiti along the underground passages at Shinjuku Station a long time ago, but we rarely see them now.