Coronavirus UK: What do the tiers mean and can I see my family at Christmas?

In tier two of England, the restrictions mean a ban on households mixing indoors and pubs, while tier three measures mean a ban on households mixing, except in limited circumstances such as parks. https://www.dailymail.co.uk/news/article-8990363/Coronavirus-UK-tiers-mean-family-Christmas.html?ns_mchannel=rss&ns_campaign=1490&ito=1490