Mandarin has 4 (technically 5) tones so “ma” could be any one of those 4 different pronunciations. Furthermore, there are many completely different words that have the same tone/pronunciation. You just learn through context (same way as you do with English homophones)

Mom in mandarin is 媽 (mā) whereas weed is 麻 (má). Colloquially you can also say 大麻 (dàmá) for weed.