Hello Carol -- That is a neat item. It belongs to a group of coin-like objects known as Silver Art Rounds, described on this page (click here). The piece is made of pure silver and that's where its value comes from.
To find the value of your round, go to kitco.com and look up the value of silver. This site (like all silver sites) reports the value of silver in US dollars per troy ounce. At the time of this writing, the value was about $20 per ounce, and, since your round contains one once of silver, that is its value: $20. But tomorrow it will be different. Look it up. The value of silver changes every day.
If you go to sell your piece to a silver dealer, expect him or her to offer less than $20. The difference is due to profit margin which is necessary to keep the dealership solvent. Margins of 15% are common for medium-sized transactions.
The origin of the term 'silver trade unit' is a little obscure. Various liberty-minded people and organizations have attempted to create their own currencies backed by precious metal. This does not usually set well with Federal authorities and, in 2007 the 'National Organization to Repeal the Federal Reserve (NORFED)' was shut down because its coins looked too much like the real thing.
One popular pattern on early NORFED coins included a balance scale with the SILVER TRADE UNIT inscription. Because these (and related) coins now have an intrigue angle, they usually sell for a little more than base silver price.