When visibility is lower than visual meteorological conditions, IMC rating allows you to pilot your aircraft. This gives you the skills to fly and land in IMC weather conditions. This may only be endorsed on to a UK or EASA license as IMC rating is national.
A minimum of 15 hours training with an instructor, including 10 hours flying by sole reference to the aircraft instrumentations. To achieve an IMC rating, you must pass a written theoretical examination as well as a practical flight examination. To keep the rating validated and current you will need to undergo re-flight testing every 25 months.
UK Flight Training 