The most important thing to the Founding Fathers was that the president not have absolute power. They rebelled and wrote the Constitution in response to tyranny, and wanted to ensure that other branches of government could check the executive branch so that the president was not too powerful.
To expand on the idea of separation of powers: In general, the laws are made by the Congress but not carried out by them. The executive carries out the laws but cannot make them. The judiciary interprets the laws but cannot make or execute them. This is separation of powers.
Powers of government are separated among the executive, the legislature, and the judiciary. The Constitution includes a system of mutual checks and balances to keep (theoretically) one branch from becoming too powerful. A few examples of this are the president's ability to appoint members of the federal judiciary, Congress's power to impeach and remove the President or members of the judiciary, and the judiciary's power (not exactly spelled out, but implied) to rule on the constitutionality of acts of Congress and executive orders.