why mix henna and lemon when using it to dye hair? what color will my hair be after i

[just another girl]

dye it i have brown hair? i want to use copletely natural henna to dye my hair, why do we have to mix it with lemon? can i just mix it with water?
also i would like to know if i dye my brown hair what color will it be afterwards? i was hoping for a auburn thanks