Abstract thinking describes thoughts that are symbolic and conceptual and not concrete or specific. Concrete thinking focuses on the present or here and now specificity while abstract thinking is based on concepts, principles, and relationships between ideas and objects.
For example, a statement derived from concrete thinking would be "There are 3 dogs." An abstract perspective could be thinking about numbers, different types of dogs, how some animals are pets, or how wolves and dogs are related. Young children are essentially just concrete thinkers - abstract thinking develops with age.