What are the holes in ice trays called?

If you consider the ice tray to be a mould (noun definition 3; or mold for AmE) for ice, then the technical term would be a cavity.

Mold Cavity Hollow space, or cavity, in the mold, which is used to impart the desired form to the product being molded. - Engineering Dictionary

Here's one for sale on Amazon that describes the holes as cavities, and here's another I found on the internet. (I'm not knowingly affiliated with the sellers).

they are called cells.

Visualize an upside-down ice cube tray, which is called an evaporator, placed upside down atop a molded water plate. The KOLD-DRAFT evaporator is refrigerated and has individual cells. The water plate has a hole for every cell, through which the water is vigorously pumped, injecting it to the top of each cell and down the four sides. Each cell acts as an individual freezing unit. The evaporator is solid copper for the most efficient heat transfer and Electrotin plated for protection against corrosion. Tin also has anti-microbial qualities and has been used to protect food and potable water for many years.

http://www.kold-draft.com/ice-making-technology/our-ice-making-technology.php