What is Jesus referring to as trading in the parable of the talents.

First of all, what is a “talent”? The English Standard Version Study Bible notes say that in Old Testament times a talent was a unit of weight equalling about 75 pounds (34 kg.).

In New Testament times, it [a talent] was a unit of monetary reckoning (though not an actual coin), valued at about 6,000 drachmas, the equivalent of about 20 years’ wages for a labourer.

In the parable of the talents, the master of the house is clearly entrusting money to his servants. The New Living Translation says the master of the house “gave five bags of silver to one, two bags of silver to another, and one bag of silver to the last” (Matthew 25:15). If each bag (or talent) weighed 75 pounds (34 kg.) and contained silver coinage, then the value of each talent would have been considerable.

When used as a measure of money, it refers to a talent-weight of gold or of silver. In June, 2018, the international price of gold was about US$41,155.69 per kilogram. One gram costs about $38. At this price, a talent (33 kg) would be worth about $1,400,116.57. Similarly, in February 2016, the price of silver was about $15 per troy ounce or about 50 cents per gram, so a 33 kg silver talent would be worth about $16,500... The talent as a unit of coinage is mentioned in the New Testament in Jesus' parable of the talents. One talent was an incredible amount of money. Source: https://simple.wikipedia.org/wiki/Talent_(weight)

Whatever each talent was worth, the master of the house, who was going away on a journey of indeterminate time, expected his servants to put this money to good use. Upon his return, the servant who took his single talent and buried it in the ground was severely reprimanded by his master:

“You should have put my money on deposit with the bankers, so that when I returned I would have received it back with interest.” (Matthew 25:27, New International Version)

The Greek word for bankers comes from trapeza (table), a word seen on the front of banks in Greece today. Bankers sat at small tables and changed money. (Source: New International Version Study Bible notes on Matthew 25:27) The English Standard Version Study Bible notes makes this comment regarding Matthew 25:27:

“In the O.T. Israelites were forbidden from charging interest to other Israelites, but it was permissible to charge interest on money loaned to Gentiles (Deut. 23:20).”

This is confirmed in this Wiki article on banking: https://en.wikipedia.org/wiki/History_of_banking#Judaism

The same Wiki article comments that “in ancient Greece and during the Roman Empire, lenders based in temples made loans, while accepting deposits and performing the change of money.” https://en.wikipedia.org/wiki/History_of_banking

It would therefore seem likely that the master of the house expected his servants to put his money to work by depositing it with the bankers so that it would earn interest while the master was away.

A "talent" in this passage refers to a large amount of money.

Being money, it was probably traded in the finance and investment sector, used to pay wages, or used for purchasing goods in the market. But, it is not a single denomination of currency, it is an amount of money.

To be very, very slang—yet also make the point to the audience—one could VERY loosely translate it as "a suitcase of money". What can you trade with a "suitcase of money"?—a lot! (Of course, a 'talent' is more specific than just a 'bag full' or a 'suitcase full', but it was probably delivered in a bag, or something similar, since it was so much money.)

Consider this Wikipedia article...

Assuming it was the "Attic talent", one talent would have been worth about nine years of manual labor. If the "Homeric talent", it would have been only worth an ox. Most Bible teachers would lean toward the larger amount. But, either way, it was worth a substantial amount of money.

The term "talent" as a kind of natural knack for gaining a skill is not what is meant here. Some argue that using the word "talent" to refer to a skill aptitude was an application from this passage, but that would have been after the fact, applying the Bible figuratively. It is not unusual for the Bible to influence language.