Most people have a partial sense of what it means to relax.
One meaning is the relief of tension and accumulated stress. When you exercise, you integrate your slower mental processes into a more encompassing state of awareness. When you build effective forms of leisure like walking in nature into your life, you slow down your over-driven mental system. When you do yoga or meditate, you quiet your mental mind in order to access your superior states of consciousness. When you routinely do these things, you experience yourself as being more balanced, more centered, more whole, more focused and more positively energized.