What is an infiltrate?

My mom has a pleural effusion (has had before, but they've mostly resolved). She's in a SNF, had a pacemaker placed on Monday at the hospital, discharged on Wednesday. The Nurse called today to say that they got an xray, they see a pleural effusion, are treating with Lasics, an antibiotic, nebulizer and incentive spirometry. X ray report says "underlying infiltrate cannot be ruled out" Can someone tell me what that last bit means. I thought I knew, but googling around has left me confused All you RNs out there, please tell me! Thanks!