What is the difference between "a" and "á" in Portuguese?

أَرْسِلْ رِسَالَة

What is the difference between "a" and "á" in Portuguese? Basically just the sound of them. "a" is the definite article whereas "á" is never alone, it is always in a word, like " água, hábito". The sound of "á" is more open than the sound of "a". Watch the video and practice the difference of the sounds with me! :)