What is a trade silver dollar?

They were actually just called "trade dollars" to distinguish them from normal circulating Morgan-design silver dollars.

Trade dollars were slightly heavier and were minted for use overseas where they competed with special trading coins that were issued by various countries. The major powers were vying for economic supremacy and a standard coin that would be accepted in other parts of the world was one way to help "smooth the way" for such commerce.

Enter the term "trade dollar" (with the quotes) into google, Ask, or any other decent search engine for oceans of information.