Ideogrammic compound (會意): 日(“sun”) + 甲(“first”) – when the sun first rises.
In some late bronze inscriptions, it was also written as a phono-semantic compound (形聲, OC *ʔsuːʔ): semantic 日(“sun”) + phonetic 棗(OC *ʔsuːʔ), with 棗 sometimes simplified as 朿. In ancient texts, 蚤 (OC *ʔsuːʔ) was often used instead.
Alternatively, it is also possible that 早 was originally a pictograph of a particular type of acorn. The shells of these acorns would be used to make a dark-colored dye. This color indicated the darkness before sunrise and the symbol then took on the meaning of early or fast. (Can this(+) etymology be sourced?)