They are both Romance languages, meaning they developed from Vulgar Latin spoken in the Roman Empire. However, they evolved independently over centuries, leading to significant differences in pronunciation, vocabulary, grammar, and even writing systems.
While Portuguese and Spanish speakers may understand some words and phrases from each other's languages, they are not mutually intelligible.
Think of it like English and German – they share a common ancestor (Proto-Germanic) and have some similarities, but they are distinct languages.