TLV is Tag-length-value encoding. Often it is better referred to by it’s original name, type-length-value.
The first field is the “type” of data being processed, the second field specifies the “length” of the value, the third field contains a “length” amount of data representing the value for the “type”.
Multiple pieces of data can be transmitted in the same message by appending more triplets to a previously existing message.
There’s a page on wikipedia covering it in just a little more detail. Don’t get confused though, each triplet is a “top level” description, there is typically no nesting of items in TLV (although you could come up with a way to do so by encoding TLV triplets in the V of another tag).