The NHS describes homeopathy as follows:

Homeopathy is a “treatment” based on the use of highly diluted substances, which practitioners claim can cause the body to heal itself. 
2010 House of Commons Science and Technology Committee report on homeopathy said that homeopathic remedies perform no better than placebos (dummy treatments).
The review also said that the principles on which homeopathy is based are “scientifically implausible”.
This is also the view of the Chief Medical Officer, Professor Dame Sally Davies.

What is homeopathy?
Homeopathy is a complementary or alternative medicine (CAM). This means that homeopathy is different from treatments that are part of conventional Western medicine in important ways.
It’s based on a series of ideas developed in the 1790s by a German doctor called Samuel Hahnemann.
A central principle of the “treatment” is that “like cures like” – that a substance that causes certain symptoms can also help to remove those symptoms.
A second central principle is based around a process of dilution and shaking called succussion.
Practitioners believe that the more a substance is diluted in this way, the greater its power to treat symptoms.
Many homeopathic remedies consist of substances that have been diluted many times in water until there’s none, or almost none, of the original substance left.
Homeopathy is used to “treat” an extremely wide range of conditions, including physical conditions such as asthma and psychological conditions such as depression.

There is more information about homeopathy, and what you can expect from an appointment with a homeopath, on the NHS website.