Slimming world is a diet plan which involves (or can do) going to a group weekly for weigh in, inspiration and support. The groups (or classes) are lead by a consultant.
The diet consists of free foods (meat, fish, veg, carbohydrates) and healthy extras (bread, cereal, milk, cheese) and syn values items (chocolate, alcohol) - all are examples, there are other things you can have!
There's a Slimming World for Newcomers thread in the sticky section which explains it further.
If you've never done it before then it's a good idea to go along to a group and have the consultant explain the plan to you. There's no obligation to join, you can sit through the group and decide at the end.