The Maple Leafs were looking for a defenseman. They got probably the best one still available in Mark Giordano from the Seattle Kraken. The trade details have not been completely revealed yet, but two second round picks (2022 and 2023) and a third round pick (2024) are going to Seattle.
The most important addition here is obviously Mark Giordano. He was named the first Kraken captain in history, but obviously, his adventure in Seattle didn't last long.