The Italo-Dalmatian languages are a subgrouping of the Romance languages made up of languages largely spoken in central-southern Italy and (historically, and very marginally to this day) what’s now coastal Croatia. The dividing line between these languages and the Gallo-Italian languages to their north (and to the entirety of Western Romance) is generally taken to be the La Spezia-Rimini line, which divides the languages that pluralise by adding -s from the languages that pluralise by changing the final vowel.
The most prominent member of this group is Italian. A number of other well-known languages of the Italian peninsula, like Neapolitan and Sicilian (along with not-so-well-known languages, of course) are also members of this group. Venetian is argued by some to be Italo-Dalmatian, but by others to be Gallo-Italian, and by others still to just be its own thing.
The second part of this group is the Dalmatian language(s) (I’m not sure how different the different varieties were from each other), which were spoken in a number of cities along the Croatian coastline for many centuries, even as the surrounding land became Slavic-speaking. Until relatively recently it was believed that Dalmatian had gone extinct in the nineteenth century, until linguists analysed the Istriot language spoken by 400 people in two villages of Istria, Croatia, and identified it as a variety of Dalmatian.
Overall, the Italo-Dalmatian subfamily is a fairly conservative group within the Romance languages, certainly phonetically, and some Italo-Dalmatian dialects have even retained the old Latin neuter gender (and in some cases divided it into two genders!) as productive word classes.