Meaning of 'type-safe'??

It means you define a variable to explicity hold data of one type and it can't store anything but that type of data (mostly).

For example in a language like php which isn't type-safe you can have
$p = "hello"
$p = 123;

Whereas say in Java you have

String s;
s="Hello";
s = 123 (can't do this).

Well technically that's strong-typing but I think type-safe is roughly the same thing.

If it's type-safe you can also only perform actions on a data type which were meant for that data-type. For example if you're collect has a String in a[1] and an Integer in a[2] then you can do a[1].length() but not a[2].length()