They were using a large wooden mallet to pound cooked glutinous rice into mochi. When it's finished it's a sticky white mass that is used in all sorts of traditional sweets. This is the time of year for mochi making and all sorts of confections featuring mochi are available in stores.
Of course mochi can be preserved and is available year round in blocks that can be softened by heating in toaster ovens or microwaves or under the gas grill. Mochi is also an ingredient in the traditional New Year's soup called zoni. Every year people are reminded to be careful when eating mochi. It is so difficult to bite a small bit off the mass of mochi to be chewed and swallowed that a person often gets more than they bargained for. It's quite easy to choke on it!
These sweet sisters were enjoying their treat as were many others.