You get an extra six months of time to file your taxes if you get a tax extension. Simply put, you now have until October 15th to file your completed return instead of the original April 18th deadline.
If you owe money when April 18th comes around, then you'll need to file Form 4868 by April 17th to avoid paying penalties on your overdue tax balance. If you do not owe any money to the IRS, then it's worth considering filing an extension just for peace of mind. But remember, extensions only give you six extra months past the original deadline of April 18th - so plan accordingly!