In this context, "cure" is a chemical term. Definition from Wikipedia :
Curing is a chemical process employed in polymer chemistry and process engineering that produces the toughening or hardening of a polymer material by cross-linking of polymer chains. Even if it is strongly associated with the production of thermosetting polymers, the term curing can be used for all the processes where starting from a liquid solution, a solid product is obtained.
So here it means that the slides were incubated at room temperature for 24 hours to let the mounting media harden. Of note, not all mounting media harden, some may require specific conditions to cure like UV light (example), and the curing step changes the properties of the mounting media like the refraction index. From the ThermoFisher website :
Formulations of mounting media that can add favorable properties such as optimizing the refractive index to match that of glass, preventing photobleaching, or preserving samples for long-term storage are widely available. Keep in mind that some require time to “cure” or harden. For mounting media that need time to cure, it’s important to let the sample fully harden before imaging so that you don’t inadvertently damage or destroy your sample by moving it around on the slide or cause photobleaching of your stained cells. Typically, the refractive index of the mounting medium will not reach its specified value until after it has fully cured, and its photoprotective properties will increase during the curing process. The degree of hardness after the mounting medium has fully cured will also vary; some will set like jelly while others will set like hard plastic