When buying a house, your deposit becomes home equity after you complete the purchase.
If you take out a Help to Buy: Equity Loan, this won't become equity until it is paid back. The loan is worked out as a % of the market value, so will cover the same % of equity until it is partly or fully paid off. This means the amount owed could change depending on changes to the value of the property.
The loan will, however, reduce your mortgage 'loan-to-value' (LTV) - the percentage of the value of your property that is mortgaged.
So, for example, if you place a 5% mortgage deposit and take out a 20% Help to Buy: Equity Loan, you will have 5% equity once you complete the purchase, and the loan will cover 20%. This leaves a 75% 'loan-to-value' (LTV) mortgage.