Roles of a historian
At its most simple, the role of the historian is to read records like this, report on what he or she finds there, and thus inform the wider world about the past. The most basic forms of history might cease at this point - simply arranging a report of events into a chronological order, and providing no further comment or discussion. We usually call such histories 'chronicles', and although they were quite common in the middle ages, they are not (with certain exceptions) the kind of history we are used to today.
'... historians treat their sources with fidelity (that is, do not pretend that the records say things that they do not ...'
So what else does the historian do? What roles does he or she play? There are a variety of things, some overlapping and common to most historians, some more divided and particular to individual historians.
There are some shared skills and methods that all reputable historians deploy. For example, they treat their sources with fidelity (that is, they do not pretend that the records say things that they do not, and do not deliberately ignore records that contradict the historian's argument).
The task of the historian, however, is more complicated than that of simply reporting what the records say. At the very least, the records that survive for most periods of history are both incomplete and often contradictory, and the historian therefore has to try, in some fashion, to address those gaps and contradictions. That is, he or she has to act as an interpreter.