If you're observant, you’re paying close attention to the details. If you're really observant, you would notice the little blob of mustard on your boss's jacket and brush it off for him before he goes to his business dinner.
Your mother may be so observant that she notices every bit of dust you miss when you vacuum. But if you're so observant that you notice every coin on the sidewalk, you might soon have enough money saved to buy your mom a robo-vac. Observant can also mean that you're careful to follow laws and customs to the letter. If you’re an observant Jew or an observant Catholic, you’re following your religion’s rituals and practices very strictly.