Prestige & Reputations are two concepts that affect how your cars are perceived by the potential buyers. Throughout the campaign you can gain or loose Prestige or Reputation in a various ways. Both of them have a maximum value of 100 and minimum value of -100. Those values are accumulated independently for every market. You can have high Prestige in one country, while low Prestige in another one. You can check those values in the Marketing & Dealerships view.
If you sell reliable cars your reputation goes up, if you sell prestigious cars your company prestige goes up. You can increase Prestige
For example if you have 1.1 build quality and 20 prestige, your prestige is actually going down.
Producing car locally or having head quarters in the given country gives a boost to the Prestige & Reputation in that country. On the other hand, foreign companies have a negative modifier.
Dealership Happiness is also affected by the effective Prestige & Reputation by the following formula:
(reputation + prestige + (max(reputation, prestige))) / 3