kagami-mochi 鏡餅 is “mirror” mochi displayed in the house during New Years. You are not supposed to open it and “break” the mirror (crack the mochi into pieces) until Jan 11th (some regions might do different days, like the 15th).
I cannot tell you the number of Americans who buy one, open it, and BITE into it raw! Then they wonder why Japanese eat wax. Gross. Please do not do this.
You must heat up the dried mochi in some way: toaster oven, microwave, grill.
On kagami biraki 鏡開き (open/break the mirror ceremony), open up your dried mochi that you bought and break it into pieces (you should not use a knife but mallet or something instead, but I won’t judge you if you decide knife is easier). Inside your plastic container you will find kiri mochi 切り餅, rectangular shaped dried rice cakes. You can buy just packages of these kiri mochi in the stores as well (sometimes you can find them in round shapes as well, those usually these are labelled with maru 丸 meaning round).
My favorite way to prepare it is to toast it in the microwave or toaster oven (just a little until you see it puff up and brown), then add it to hot zenzai ぜんざい (red bean soup, you can make yourself or just buy the prepared package at SanA). Something similar is oshiruko おしるこ, which is more soup-y and smooth.
Another way would be ozoni soup お雑煮, which is usually simple dashi, mochi, plus some vegetables or fishcake.