This is used to modify the IPv6 HOPLIMIT header field. The HOPLIMIT field is similar to what is known as TTL value in IPv4. Setting or incrementing the HOPLIMIT field can potentially be very dangerous, so it should be avoided at any cost.

Don't ever set or increment the value on packets that leave your local network!
    mangle table.

--hl-set value
    Set the HOPLIMIT value to `value'.

--hl-dec value
    Decrement the HOPLIMIT value `value' times.

--hl-inc value
    Increment the HOPLIMIT value `value' times.