High-bar back squats require that the bar is resting on top of your upper back, or your traps. High-bar squats are typically easier to perform, place less pressure on your wrists, and place more emphasis on quad engagement (muscles on the front of your thigh).
A low-bar back squat requires that the bar is held lower on your back, right below your rear delts, which requires a more wrist flexibility. The low-bar squat recruits more hamstrings, glute, and hip muscle activation. You will most likely be able to lift more weight with a low-bar back squat than high-bar. Either squat variation is acceptable, so choose whichever is most comfortable for you.