What is Calldata tag needed when a function has a string parameter?

Question

I'm using:

 pragma solidity ^0.5.0;

---

I have a function as below in my contract:

function test(string a) external{
  a="apple";
}

An error shows up saying I need to add "calldata" to the parameter. So, if I do the following the error disappears:

function test(string calldata a) external{
  a="apple";
}

Question: Why do I need to add "calldata" when I use string type as a parameter?

Answer 1

Explicit data location for all variables of struct, array or mapping types is now mandatory. This is also applied to function parameters and return variables. For example, change uint[] x = m_x to uint[] storage x = m_x, and function f(uint[][] x) to function f(uint[][] memory x) where memory is the data location and might be replaced by storage or calldata accordingly. Note that external functions require parameters with a data location of calldata. for more Info