The gauge of guitar strings refers to the thickness of each string. The lower the gauge number, the thicker the string. 9-gauge strings are thicker than 10-gauge strings.
Whether 9 or 10-gauge strings are better for a Stratocaster guitar depends on personal preference and playing style. Thicker strings (9-gauge) provide a stronger and richer tone with more sustain but may be harder to play for some musicians.
Thinner strings (10-gauge) are easier to play, but may not provide as much tonal depth and sustain.
In clonclusion, the best gauge for a Stratocaster guitar is the one that feels and sounds best to you. It is recommended to try out different gauges to find the right fit.