What if I don't do a extension

[NickyL]

You'll need to either file by the tax deadline (tonight @midnight), or file an extension to avoid paying penalty or late fees from the IRS. 

The exception to this being if you do not need to file a tax return. You can see if you qualify to not file here:  https://turbotax.intuit.com/tax-tips/irs-tax-return/does-everyone-need-to-file-an-income-tax-return/...

Here's a helpful video as well: https://turbotax.intuit.com/tax-tips/general/video-who-needs-to-file-an-income-tax-return/L34QqaiCJ