Being heedful means paying careful attention to something (or someone). You'll want to be especially heedful of your best friend's feelings if he's having a bad week.
If you're heedful, you're mindful or attentive. You should be heedful of drinking plenty of water when you work outside on a hot summer day, and heedful of the crumbling stone steps if you're exploring the ruins of an old building. It's also important to be heedful of the way other people feel. This adjective comes from heed, "careful attention," and its Old English root hedan, "observe or protect."