If you are applying for a green card in the U.S. through the procedure known as "adjustment of status" (having submitted Form I-485 to an office of U.S. Citizenship and Immigration Services), and you thought you might like to travel while awaiting your green card interview, you probably submitted Form I-131, requesting Advance Parole (AP). The AP document gives applicants the right to leave the U.S. and return without facing the cancellation of their application (as would occur if they failed to apply for AP before leaving.)
But you may have also read warnings in Nolo's products and from USCIS saying that if you had accrued unlawful presence in the U.S., you should not leave even with an AP document -- it would not protect you from being found inadmissible upon your return. Unlawful presence is a ground of inadmissibility. If you accrue more than 180 days unlawful presence in the U.S. and then leave, you can be barred from returning for three years. A full year's accrual of unlawful presence gets you a time bar of ten years.
Good news from the Board of Immigration Appeals (BIA): You can pretty much disregard those warnings. In the Matter of Arrabally and Yerrabelly, Interim Decision #3748, the BIA held that departures under Advance Parole with a pending adjustment of status application do NOT trigger the unlawful presence bars.
If you didn't submit an application for AP with your adjustment application, you can still do so. Download Form I-131 for free from the USCIS website. Be sure to submit a copy of your Form I-797 receipt notice for your adjustment application along with the form. That will allow you to avoid paying a separate fee to file it.
Check with an immigration attorney if you have any questions about whether this applies to your situation.