Hm, a bit more to it than that. But I'm not good at explaining this sort of thing.
"Traits" are more like personality aspects, things like that. They simply "are", and technically can exist without autism to the same degree. Autism may make those things far more likely to be present, but it's not necessary. They can be positive or negative.
"Symptoms" are direct effects that pretty much define what autism is to most people. Sensory meltdowns, inability to understand social norms, stuff like that. They tend to be "heavy" things. It's sometimes possible to have them without the condition, but in that case it's usually fairly minor (and when it isnt minor, it's probably associated with a different type of condition). For those on the spectrum, it's drastically worse and can really wreck you at times. These are rarely positive.