What makes tDCS work?
Let us start with the application of DC current and its study at neuronal level. While the electrical current goes through the scalp and cortex it encounters different mediums. Each time it goes through the limit between two mediums it leaves some charge behind. This is what happens when traversing the neuronal membrane, which constitutes the interface between the interior of the neuron and its environment. These charges generate a voltage difference, which makes the resting potential of the membrane either decrease or increase, depending on its sign, i.e. on the current sense. Therefore the neuron is correspondingly either more likely or less likely to fire. When applying an anodal current, the action potential thresholds of pyramidal cells under the anode application area are lowered, meaning that they fire more easily. Therefore this current, and its associated electrical field, is considered excitatory. On the other hand a cathodal current would present an inhibitory effect.
via Quick and Dirty Guide on Transcranial Current Stimulation.