it is an Egyptian dialect, It means "Nothing".
Its meaning differs from one context to another,
When someone asked you ın Egyptıan dialect: "what happened ?!" "حصل ايه ؟ "
and you want to say, "Nothing happened!", then you reply with: "مفيش "
also if you asked a vendor or a salesclerk if a product exists in the shop,
he would reply "لا مفيش" it means "No there is not".
Also if you wanted to say "There is no one her" = "مفيش حد هنا"