The Help to Buy equity loan scheme is a government first-time buyer scheme that requires a minimum 5% deposit and after which the Government will provide you another 20% ( or 40% if you are buying within London) in the form of a 5-year interest-free loan(after which it is 1.75% per annum). You can only get the help to buy equity loan with a standard repayment mortgage, not a buy to let mortgage or commercial mortgage.
If you want to see how much you could potentially borrow with the help to buy equity loan then you should consider using a help to buy calculator.
This interest rate then rises each year by the retail prices index(RPI) inflation measure + one percentage point.  Assuming RPI is 4%, the interest rate would rise by 5% (4% + 1%). It is important to note that you only pay the interest from year 6 and not the capital repayments itself. This means you will be required to make monthly interest repayments on your help to buy equity loan from the 6th year.
The Help to Buy equity loan is only available for new builds. In England, it’s only available for those buying a new build worth less than £600,000 (£300,000 in Wales, and £230,000 in Scotland). The scheme was set up to increase the supply of affordable housing in the UK. The help to buy equity loan is competitive as the 5% + 20% deposit puts you in a better Loan to Value band and hence a cheaper mortgage. The Help to Buy equity loan scheme is accepted as an addition to your mortgage deposit by most mortgage lenders in the UK.

The Scottish Government are more generous and will never charge any interest but rather they will retain 20% of the property value when the property is sold.
This means that if property prices rise the government can make some money from that rise and vice versa.

You can pay back some or all of your equity loan without selling your home. You can make a partial payment called ‘staircasing’. This will only be accepted if it is 10% of the current property value or you are clearing the whole of the mortgage with the lender.

**Example of the help to buy equity loan. **
A home costs £400,000. You provide a 5% deposit of £20,000 and the government loans you another £80,000(20%). You then take a mortgage of £300,000(75%). The government loan effectively pushes down your LTV and ensures the loan is affordable.
Tip: combine the Help to Buy Equity loan with the Help to Buy ISA for maximum effect