The system of checks and balances is the system where all three of the US government branches are able to check one another to ensure that no branch has more power than the other.
This ensures safety for the citizens of the country to prevent government corruption. To illustrate, if the executive branch had the most power, the president of the US would be able to make executive orders as he pleased whether or not it is constitutional. To prevent this, we have the judicial branch ruling whether or not a president's actions are constitutional and we have the legislative branch holding the right to impeach the president in the case that he/she does something illegal.