The cabinet is a group of politicians who have really important jobs. They meet about once a week and work out how to run the country. A cabinet reshuffle is when a prime minister changes the members of this top team. Prime Ministers can reshuffle the cabinet to reward people by promoting them or remove people who are not doing very well. They can also reshuffle the cabinet to show that they have changed their policy direction or priorities.
Reshuffles are more common in parliamentary systems where the executive is drawn from the legislature, than in presidential systems that separate the executive and legislative branches of government. This is partly because reshuffles are easier in parliamentary systems, where there is a larger pool from which to choose ministers for specific portfolios.
In some countries, the principal needs to get approval for ministerial appointments from the legislature, which adds a layer of formality and oversight that reduces the incentives to regularly reshuffle the cabinet. In other countries, there are rules limiting the number of times that a cabinet can change its composition during an electoral cycle.
The frequency of reshuffles varies considerably across the world. It is a common practice in parliamentary democracies and is far less common in constitutional republics, where the constitution might prohibit the principle from moving around members of his or her cabinet. It is also more common in governments with fewer ministerial portfolios, where each portfolio might be more easily overseen by an individual minister.