This paper analyses the importance of demand-side flexibility as part of the energy flexibility necessary to manage the power system by the Transmission System Operators (TSOs) and increase power systems market efficiency. European countries set a significant challenge in their goals to achieve carbon neutrality. The pivotal shift from traditional, centralised power generation towards a more decentralised and flexible system is required to meet the challenge. Traditional centralised coal and gas power plants supplied markets with reliable power delivery and a wide range of control necessary to balance power grid needs when there is a disbalance between supply and demand. With the attenuation of the carbon-heavy coal power plants and the shift to renewable energy sources, like wind and solar power plants, new challenges exist with conditioned production for the markets and a reduction in balancing resources for the power grid. Addressing these challenges optimally requires focusing on both supply and demand. Thanks to the rapid advancements in technologies and control mechanisms, it became possible to have a more active role on the demand side. Increased variability in production has led to fluctuating hourly prices, offering advantages to entities that can modify their energy consumption and production patterns. The aggregation of smaller production units and energy systems allows smaller participants, through aggregators, to participate in energy and balancing markets and capture additional value. This paper provides a comprehensive introduction and overview of this crucial topic.