träffas has reciprocal meaning, it means that it doesn't require an object because the subject of the verb is doing the action to 'itself' (= each other). träffar is a normal verb that doesn't make sense without an object. You can't just say Vi träffar in Swedish, you'd immediately be asked who you're meeting. So Vi träffas means the same as Vi träffar varandra.
It's not possible to say you're meeting someone without indicating who, at least grammatically (you'd have to say e.g. 'någon', and then there would be an object). träffas means the people who are doing it are meeting each other. But if there is an object, you use träffar, so your sentence is ok.
Like: jag träffar min mamma or jag och min mamma träffas.
I can't think of anything reciprocal, but a sentence in the course where the English verb needs an object but the Swedish doesn't is Hon tackade för maten which translates into 'She said thanks for the food' – or you could add who she thanked, but in Swedish you don't need to mention them. So you can say Han tackade och gick literally 'He thanked and left' which of course doesn't work in English.