This window appears when someone is offering you a trade in a network game.
The offer itself is pretty straightforward. What the person is offering is on the left of the window, what they want from you is on the right. Your possible responses are the buttons on the right of the screen. Once the trade offer window has appeared, a timer will start counting down, shown in the text in the top of the window. Once this timer gets to zero, Sea3D will automatically reject the trade for you. This is to prevent the game from being halted if you step away from your keyboard for a bit. The auto-reject timer length can be set by going to the General Settings window from the Options->General menu.
If you have what the player wants, and you want what they have, click the Accept button to let the player know you're willing to deal. If you do not have what they want, the Accept button will be disabled.
If you do not want to trade with this person, click the Reject button.
If you are willing to trade but want to specify your own offer, click the Counter button. The window will expand to become the window you see below.
Once the window is expanded, you can alter the trade to be what you would like to offer to the player. Click the cards in your hand to add to your offer and click cards in the Cards to ask for section to add to what you want from the player. Once you have everything set, click the Offer button to send the counteroffer. Alternately, you may still reject the trade completely by clicking the Reject button.