Property and field

Hi,
     I am a bit confused over property and filed..
I come across this on a website..
public class Customer
{
private int m_id = -1;

public int ID
{
get
{
return m_id;
}
set
{
m_id = value;
}
}
}


As an advantage of property over field,they had mentioned

"For example, if the type of
m_id
changed from an int type to byte, calling code would still work"


Can anyone explain this with an example...

...