Definition of cold
Literally, "cold". The term cold is often used at the poker tables in a variety of contexts. You can be "cold deck", which means that you haven't received a decent hand in a long time. And then there are the expressions "cold call", "cold 3bet", "cold 4bet", etc. What is usually meant by "cold" is that one does not close the action. For example, a conservative player who hasn't played a hand for a long time and opens in the UTG position. A player on the cutoff 3bet opens and you, on the small blind, over-roll (4bet). In this situation, we'll say it's a "cold 4bet".
Implications of the cold 4bet
There are no absolutes in poker, but let's just say that generally speaking, the expression "cold 4bet" in our example above indicates a lot of strength. If we take our example from the small blind, we go with a cold 4bet, that is, we don't close the action. The UTG player can have a very big hand, the player who has 3bet can have a very big hand. The player who "cold 4bet" doesn't know what the other players are going to do and has no clue that a player is weak. In summary, "cold" is used when a player does not close the action. If our hero above had called instead of 4bet, it would have looked like "cold call 3bet".