Answer:
To put it simply, Tosh argued that collective memory of a certain group can be faulty often doesn't represent the full truth because of the different perspective that this group experience.
For example,
Let's examine the colonization that happen in the past. If you based the history on the European perspective, their collective memory probably support their cause because they believe they are "teaching" the colonized countries to be more developed.
But, if we see the exact same event from the colonized country's perspective, their collective memory most likely stated that they were invaded and large portion of their resources were taken. This eventually contribute to the stunt of their growth.
Because of this, on way forward to review historical event would be considering the individual perspective of all the parties involved in each event. The truth usually lies in the middle.