I heard the term "dead draw" a lot, but I don't know its meaning and when a position is "dead draw".
A "dead draw" position is an endgame neither side can make further progress. It's not an immediate forced draw, meaning that the "stronger" side does not have to play a line that leads to stalemate or insufficient material, three-fold repetition or anything like that. However, all attempts they have to win can be stopped with an accurate defense that shouldn't be too hard to find. Eventually, the side trying to win will have to accept the draw or reach the 50-move rule.
It's not always easy to draw a boundary for what counts as "dead draw". The most useful definition is probably "a position where neither side can win where one of the players could make the game last forever (without the 50-move rule)"
Fortresses are an example of dead draw position. Positions with all pawns blocked are often, too. The Philidor position is a basic theoretical dead draw. Opposite-colored bishops often lead to a lot of dead draw endgames.
The answer is when neither of the players can proceed further, both players are playing fairly well but the stipulated number of moves by both players have ended when one player who must exercise onus with moves, play his pieces a queen, a Bishop or rook with moves perpetually the same moves again and again to avert a possible check, and especially stipulated time and moves as per FIDE rules have come to an end. This is a dead draw in a chess tournament.