it means sticking a finger (preferably wearing a glove) into a stoma.
Should always do this as part of your exam if you have a patient with a stoma that has a bowel obstruction - to determine if the obstruction is at the level of the stoma (often is). Never force it if it doesn't go easily or if the patient has discomfort. Dont' do on a fresh stoma unless you know what you're doing.