How does heat affect resistance?
Well, consider a piece of conductor and think of it as a long corridor where, at fixed intervals and places, you have persons standing. These are the atoms that form the lattice of your substance. They are fixed but they can oscillate according to the amount of energy present in the piece of conductor (internal energy).
Now imagine that you have kids running from one side of the corridor to the other. They are the free charges responsible for conduction in your conductor. They have to run in between the "adults" trying to avoid them and in doing so continuously changing direction (so they find it difficult, they experience a resistance to their path!).
Now imagine that you, through heat, give energy to the material. Internal energy increase and the atoms start to oscillate with larger amplitudes becoming serious obstacles for the "kids" that are trying to run through them!
The resistance the kids experience increases because now the adults are bigger obstacles and it is easier to bump into them!