What does this lines mean in RFID UID Card Reader Code?
The code formats UUID as a string of 4 hex bytes (each bytes a pair of hex digits) separated by colons.

String ID = ""


Blank the string

for (byte i = 0; i &lt; 4; i ++){


Iterate over the four bytes of the UUID

  ID +=


Add the following to the string "ID"

(rfid.uid.uidByte[i] &lt; 0x10 ? "0" : "") +


If the hex number to be displayed is less than 0x10, add a leading '0' to the string 'ID'. This ensures the UUIDs are all aligned and all bytes within the UUID are displayed as two hex digits (even if only one is needed). It uses the ternary condition operator, which is a compact if/then/else that can only return one of two values (rather than executing chunks of code). See https://en.wikipedia.org/wiki/%3F: for more on the ternary condition operator.

 String(rfid.uid.uidByte[i], HEX) +


Add the UUID to the string 'ID' as HEX.

   (i != 3 ? ":" : "" );


Another instance of the ternary condition operator. This one adds a colon to the string 'ID' after the hex byte unless the index is 3 (ie the last one). 

 }


End of for loop.
This means that the result is nn:nn:nn:nn instead of nn:nn:n:nn: (leave off trailing colon and differing numbers of hex digits).