One penny coins of this type are issued by the Royal Mint as "non-circulating legal tender" (NCLT) - i.e. for collectors, and are not intended for circulation.
This coin was issued to celebrate the birth of His Royal Highness Prince Louis of Cambridge, the third child and second son of Prince William, Duke of Cambridge, and Catherine, Duchess of Cambridge.
The Royal Mint says:
"As Their Royal Highnesses The Duke and Duchess of Cambridge welcome their third child, we are delighted to capture this historic and happy moment with a silver coin.
Just as we celebrated the births and christenings of His Royal Highness Prince George of Cambridge and Her Royal Highness Princess Charlotte, the lucky silver penny struck for this royal birth is a charming way for you to mark this joyful occasion. It embraces the age-old custom of giving silver to a newborn baby to wish them good fortune in life. And if you, or your loved ones, are welcoming or christening a little prince or princess in 2018, this is a perfect way to mark the moment.
This is proving to be such a special year for the Royal Family, with many happy occasions to celebrate in 2018. Be a part of the festivities with this sterling silver penny finished to Brilliant Uncirculated standard. As a commemorative coin from The Royal Mint, your silver penny is also an official United Kingdom coin." | 
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