I'm trying to understand Maxwell's 4th equation, which is
$$ \bigtriangledown \times B = \mu_0j + \mu_0\epsilon_0\frac{\partial E}{\partial t} $$
I understand that $\bigtriangledown \times B$ corresponds to curl of the magnetic vector field. I know $\frac{\partial E}{\partial t}$ is another 3d vector representing the change in a vector field over time.
But what I'm trying to understand, is $j$. Is it just change in charge density over time? Can a stray electron moving through space be understood to have current, or does it only have meaning in circuits? And if the latter, what precisely is required for a circuit?
Essentially, what is meant by "current"?