Citizenship by descent is a process to grant citizenship to a person who was not born in a particular country but has acquired the citizenship as one or more of the parents or ancestors belong to that country.
Many countries in the world offer citizenship by descent, based on various factors. So, for example, a British citizen by descent is a person born outside the UK and is acquired if one or both parents are British citizens. This means that you cannot automatically pass on British citizenship to any child who is born abroad.
Here is an interesting infographic listing countries that offer citizenship by descent. Go through the the list and see if your parents or ancestors made you eligible for a dual citizenship.
An infographic from FreePeopleSearch.org