If you have already filed Form I-751 but your petition has not been approved yet, you can still safely travel abroad on the condition that you received from USCIS a receipt letter called I-797, Notice of Action. This document will extend your residence for a period of time while your application is pending, usually 1 year.
After your conditional green card has expired and before your Form I-751 is finally approved, it is obligatory to carry the following documents with you when traveling abroad: valid and unexpired passport and I-797, Notice of Action, along with expired conditional green card. When reentering the USA these documents will be a proof of your resident status and a proof that you have not abandoned your conditional residence.
Generally, you should not risk and take lengthy trips outside the USA. If your I-751 petition is denied while you are abroad, you may not be able to reenter the United States. So if you plan to take a long trip, consider consulting an immigration attorney and obtaining a reentry permit.
Also, if you have criminal offenses history or some document issues consider not leaving the US till the procedure is over and conditions on residence are removed. Note that in some cases it is easier to visit other countries first and then to submit Form I-751 when you return to the US.