The three branches of government are set up so that each branch of government has a defined role and that the roles are divided amongst the branches. This is known as separation of powers. So, in practice, the legislative branch might create the laws and the executive branch would execute those laws via agencies. Or the Executive branch might nominate a Judge but the legislative branch has to approve the Judge.
Checks and balances ensures that power remains evenly distributed amongst the three branches of government by giving each branch roles and limits to their roles.
