When you differentiate a variable ([imath]y[/imath]) with respect to another variable ([imath]x[/imath]), but the first variable ([imath]y[/imath]) is not the subject (not isolated), you use implicit differentiation.
For example:
[imath]y = 4x^2[/imath]
Here [imath]y[/imath] is the subject, means it is isolated alone. In this case you differentiate normally.
Another example:
[imath]y^2 = 4x^2y[/imath]
Here [imath]y[/imath] is not the subject, means it is not isolated alone. In this case you use implicit differentiation.