"Reference resources" are information resources that are intended to be read a portion at a time, as needed. Encyclopedias and dictionaries are great examples. When you want to find an explanation of an unfamiliar concept, you can look it up in an encyclopedia or dictionary, read the part that is about the concept, and then set the encyclopedia or dictionary aside until you need it again.
Reference resources exist in print and electronic formats. When they are electronic, they are often gathered into reference resource databases - online collections of encyclopedias and similar materials.