Defined as an English word by Merriam-Webster - without saying whether it is singular or plural - as “1. formal prenuptial conditions or agreement made at a Jewish betrothal ceremony, 2. a Jewish social function announcing an engagement,” tena’im are “conditions.” The term refers to prenuptial agreement to wed - officially and Jewishly.
Based on the long-standing (from the 12th century, at least) Ashkenazic tradition of two families matching their children and setting forth the terms for dowry, date of wedding, penalty for one or the other party backing out (kenas), etc., the document itself was referred to as “tenia’m.” It would contain compensation for any potential transgression of its terms and conditions. The document would be signed and read aloud, and a piece of crockery would be smashed. In some communities all guests brought crockery to smash. All of this would take place well prior to the wedding. (One source notes that in Sephardic tradition, tena’im would be executed right before the wedding).
Nowadays, young couples are returning to this tradition, making the agreement between themselves, without the rest of their family. The terms include money management, jobs, place of residence, raising children, and potential dispute resolution. In short, they focus on what really ought to be dealt with prior to such a rite of passage - the marriage itself, rather than the wedding ceremony.
Some refer to tena’im (תנאים) as an engagement contract. It is signed by the couple and witnessed by two persons, just as the ketubah, the wedding contract, will be. (There is apparently discussion and disagreement as to whether they may be the same two witnesses for both documents, and whether those same two persons can be the ones under the huppah with the couple witnessing the wedding itself.)
Some sources say that it was the future mothers-in-law who smashed dishes to symbolize their children planning to take care of each other rather than the mothers continuing that fretful task. (The sources saying this could not possibly have been mothers, in the humble opinion of your correspondent, as mothers generally do not so relinquish.) (In-laws, by the way, have a stated relationship with each other in Yiddish: they are mahutanestehs.) Some also say that unmarried women grabbed for pieces of the broken ceramics, as an omen that their own agreement would be forthcoming soon.
The ketubah is a requisite Jewish document, since wedding another is in fact executing a contract, which ought to be documented. A tena’im is not required halakhically, but has been enjoying a resurgence of popularity as couples look to plan for commonality in hectic lives - the promise of quiet Shabbatot, a peaceful home, the possible joy of children, partnership equality, and repairing the world together. And maybe having to “social distance” together for long periods, too.