An appreciated U.S. dollar with respect to other currencies, meaning that less dollars are needed to buy a unit of foreign currency.
The strength (or weakness) of the USD relative to other currencies affects trade. When the dollar is strong, imports become cheaper for Americans while exports get more expensive. This is a detriment to American businesses that ship their products out of the country.
This is the opposite of weak dollar which sees its impact in the reverse.
A strong dollar has led to accusation of the United States "weaponing the dollar". This can have a catastrophic impact upon the economies of the emerging markets.