Equity is the difference in the value of the home and what is owed on the home. There are two dynamics that cause this to grow: appreciation and principal reduction.
As the home increases in value, it is said to appreciate. Various authorities will annualize an appreciation rate based on average sales prices from one year to the next. Since appreciation is based on supply and demand as well as economic conditions, it will not be the same year after year.
If you looked at a ten to twelve-year period, some would be higher than others and there may even be some individual years that it is flat or even declined. For the most part, values tend to appreciate over time.
Most mortgages are amortized which means that a portion of the payment each month is applied to the principal in order to pay off the loan by the end of the term. A $300,000 mortgage at 4.5% for 30 years has $395.06 applied to the principal with the first payment. A slightly larger amount is applied to the principal each following month until the loan is paid with the 360th payment.
If additional principal payments are made, it will save interest, build equity faster and shorten the term of the mortgage. Using the previous example, if an additional $250.00 principal contribution was made with each payment, it would only take 270 payments to retire the loan instead of 360. It would save $69,305 in interest and shorten the mortgage by 7.5 years.
To see the dynamics of equity due to appreciation and principal reduction, look at the Rent vs. Own. To see the effect of making additional principal contributions on your equity, look at the Equity Accelerator.