What does "checks and balances" mean?

What does "checks and balances" mean?
Although each branch has its own authority, they are not completely separate or independent of one another. Instead, they are threaded together by a system of "checks and balances" that subjects each branch to a number of constitutional checks, or restraints, by the other branches. The checks and balances system was designed by the Founding Fathers to prevent a concentration of power in any one branch and to protect the rights and liberties of citizens. For example, the president can veto bills approved by Congress and the president nominates individuals to serve in the federal judiciary; the Supreme Court can declare a law enacted by Congress or an action by the president unconstitutional; and Congress can impeach the president and federal court justices and judges.

source:page 491,The Handy Politics Answer Book by Gina Misiroglu