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In the solution of this example, the author uses the following: $$d(0, P)=|r|$$
But what does that notation mean?
It reads: "the distance (denoted by $d$) from the origin (denoted by $O$) to the point $P$ is $|r|$"
$o$ is the origin, $P$ is a point, $d$ is the distance, so "distance between the origin and your particle".
Required, but never shown