HELOC rates are adjustable and always tied to the prime rate (the current rate at which banks lend to their most creditworthy customers).
Your total interest rate = the current prime rate + the margin (the markup your lender assigns you). While the prime rate will fluctuate, your margin will always stay the same.
Your margin is determined by considering a variety of factors such as your creditworthiness, debt-to-income ratios, the amount of equity in your home, etc.