ode(expression,y,x) solves the linear first-order differential equation $y'=f(x)\cdot y + g(x)$. expression is the complete right-hand-side of the equation, x and y are symbolic variables. Free constants in the solution are marked C.
Let's give an example; $y'= x y$.
Another example: $y'= -k y$.
or $y'= y \tan(x)+\cos(x)$.