This is clearly very unwelcome news and makes today a day of mixed emotions, given the superb news about the vaccine. Unfortunately the rapid spread of the new variant, and the risk it poses to NHS capacity, has meant that the vast majority of the country has now been moved into Tiers 3 & 4.
This comes into effect at midnight tonight.
Tier 4 means that you cannot leave or be outside of the place you are living unless you have a reasonable excuse. You cannot meet other people indoors, unless you live with them, or they are part of your support bubble. Outdoors, you can only meet one person from another household.
A reasonable excuse includes:
Work and volunteering
Essential activities
Fulfilling legal obligations
Education and childcare
Meeting others and care (including childcare and support bubbles)
Exercise and recreation
Medical reasons, harm and compassionate visits
Communal worship and life events
Full guidance here: https://www.gov.uk/guidance/tier-4-stay-at-home