tun= to do
"Ja, denken TUE ich an das schöne Wetter" =
" I DO think of the nice weather." is how you can understand it in this context.
The German language does not use 'to do' like English does, like to form questions for example.
You just simply place the German verb first in a sentence.
Does it rain a lot? = Regnet es oft?
But if you wanted to say: "Yes, it DOES rain a lot (emphatically!)
then you can say in German: "Ja, regnen TUT es oft!"
Verstehst du es jetzt?