Topic is an organized system of your Convos, so if you, for example, wish to sort convos by certain subjects you will use Topics.
Here is a great example on how to use Topics:
In MegMeals bot you can find restaurant recommendations, they are organized in Topics based on the city, where those restaurants are:
So if you wish to see restaurants in Soho you will click on that Topic and see Convos for each restaurant in that Topic:
Convos are also nicely organized by Topics in your Messenger Menu like this: