Owl: Most equations you have solved asked for a number. A differential equation asks for a function — something whose rates are tied to its values.
The statement dy/dx = y says: "wherever you are, your slope equals your height." Which curves obey that local rule?
Solutions usually come in families. An initial condition picks one particular member. Treating the answer as a single number, or grabbing any antiderivative while ignoring the initial condition, breaks the story.
If dy/dx = y, what shapes are even allowed?