Disrespectful words and actions are rude and show a lack of respect. If you want to "dis" someone, be disrespectful towards them.
You probably know that respectful things show consideration, kindness, and appreciation. Disrespectful things do the opposite. Swearing at your grandmother is disrespectful. Burping loudly in church is disrespectful. For a business to ignore a religious holiday could be considered disrespectful. This is a word with a lot of grey area: what is disrespectful to one person may be just fine to another. Disrespectful behavior can range from blatant rudeness to just not acting impressed or awed by something others hold sacred.