It could be different depending on the context, but the first thing I think of
is what happens when you buy a car in the States.
You go into the dealership with your awful ugly old car, and you pick out a new
beautiful car, which of course costs much more than you want to pay, and then
the salesman tells you that he can give you $500 for your old car, and he takes
it as a trade-in. They might say something like "Your trade is worth $350" or
"Your trade-in will get you $600 off", something like that, in their
advertisements.
Of course, " to trade" just means to exchange things -- I take yours and you
take mine.
As a verb, "trade in" could just mean that you do business buying and selling
that thing. For example, "His company trades in gold and silver."