Simply put, the trade block is a way for teams to publicize to each other which players they are most willing or interested in trading away.
It also allows SIM Teams to initiate trade offers to human owners.
More about sim teams.
Players on the trade block are viewable through the player search.
More about the player search.
To manage your trade block listings, go to your Team Menu > General Manager > Trades and then click the Trade Block tab.
Toggling the checkbox to left of a player toggles them on and off the trade block.
NOTE: Your trade block selections will be cleared at the end of each season.
Offers for players you've listed on the trade block will come through just like any other trade proposal.
SIM Teams are always looking for trade opportunities. When new players are added to the trade block, they analyze the players and make trade offers if they find matches to their own needs.
More about sim team trade offers.