I cannot understand this command?

This for loop syntax is really simple:

for (char letter : toCharArray()) { 
   //do stuff for each item we loop through
}

It is basically saying ok so lets make a char variable and its called letter and for each item in my toCharArray() im going t set that item value to letter.

So its like for each letter in my array i am going to for example print out the value.

So if the array held the chars (h,e,l,l,o)

It would go through like this:

set this char letter = item number 1 which is "h" then do the code in the braces then again set char letter = item number 2 which is "e" the do the code in the braces and so on until it gets to the end of the array and you have now looped through and printed each item out.

I dont think ever seen a for loop without the data type specified. Its always been:

(char letter : listName)

or (String s : myArray)