looking for video lecture on lifshitz approximations

there is a method which coputes differential equation by aproximations

and there is such thing which i cant understand
that if we find y0
and we want to find y1

then when we do that we vould go out of bounds
so we need to fixs some other bounds

i cant understand this thing
i cant find a video lecture on it
i cant imagine what bound we go out of
and what bound we fix
etc..