What it looks like
A geographical instruction.
Someone is apparently telling you to go over there.
What it really is
An expression that, in most cases, involves no actual movement whatsoever.
Ma va là! is one of those expressions proving that, in Italian, words can keep meaning something even after they have stopped meaning what they literally say.
It can express disbelief, brush off a compliment, play down a worry, challenge a statement or simply dismiss someone.
The interesting part is that the words never change.
What changes is the tone, the face, the length of the final a and, as so often happens in Italy, the hands.
Said with a smile, ma va là! can be almost affectionate.
A drawn-out ma va là… with an amused look means roughly: oh, come on.
A sharp MA VA LÀ!, accompanied by an arm pointing towards some undefined place beyond the horizon, means that the geographical component is suddenly becoming relevant again.
Examples
— I ran ten kilometres this morning.
— Ma va là!
— You look great in that jacket.
— Ma va là…
— Do you think he was offended?
— Ma va là.
— Italians eat pasta every day.
— Ma va là!
— Trust me, I know exactly what I’m doing.
— Sì, ma va là.
The rule
If an Italian tells you ma va là, don’t go anywhere just yet.
Look at their face first.