Noun: readiness to show appreciation for and to return kindness.
Gratitude means thankfulness, count your blessings, noticing simple pleasures and acknowledging everything that you receive. It means learning to live your life as if everything were a miracle, and being aware on a continuous basis of how much you've been given. Gratitude shifts your focus from what your life lacks to the abundance that is already present.
To say we feel grateful is not to say that everything in our lives is neccessarily great. It just means we are aware of our blessings.