Help to Buy is an equity loan scheme to help you buy a new home. If you have a 5% deposit and can cover 75% of the value of the home with a mortgage (or 55% in London), the Government will lend you up to the remaining 20% (40% in London) in the form of an equity loan that carries 0% interest in the first 5 years. This is available on a variety of new-build properties for first time buyers and is subject to the property price, the subsequent price cap in that region and the developer offering the option.
You can read more on the Help to Buy FAQ