Checks and balances exist to keep the executive, legislative, and judicial branches equal. Some examples of checks and balances in the Constitution include:
The president's power to veto a bill (executive check on the legislative branch)
The House's power to impeach (legislative check on <em>both</em> the executive branch <em>and</em> the judicial branch)
<em>Both</em> houses of Congress must pass each bill (legislative self-check since this branch is bicameral with both the House and the Senate)
Judicial review (judicial check on <em>both</em> the legislative branch <em>and</em> the executive branch)
The president's pardon power (executive check on the judicial branch)
These are just a few. Let me know if you have any questions.