What does it mean to have a blood clot in your liver?

My mom just had the gastric sleeve,( a weight loss surgery) 2 Tuesdays ago and she went into the hospital yesterday to make sure shes not dehydrated, she was supposed to come home today but she apparently has a blood clot in her liver? neither one of us knows what that truly means?? I would really like to know. and google is just not workin with me.