Mars is the planet of personal will. It is yang energy - how we express our drive to take action and get our needs met. It is how we make things happen, and how we express our anger and frustration when those things don't happen as we want. Mars goes retrograde every 2 years for 2 - 2.5 months. During that time, its energy is turned inwards and we are given time to review how we express our Mars. In addition, the signs it is retrograde in give an overarching theme for the retrograde period.