Van Gelder represents computationalism this way:

According to [the computational] approach, when I return a serve in tennis, what happens is roughly as follows. Light from the approaching ball strikes my retina and my brain's visual mechanisms quickly compute what is being seen (a ball) and its direction and rate of approach. This information is fed to a planning system which holds representations of my current goals (win the game, return the serve, etc.) and other background knowledge (court conditions, weaknesses of the other player, etc.). The planning system then infers what I must do: hit the ball deep into my opponent's backhand. This command is issued to the motor system. My arms and legs move as required.

In its most familiar and strike and successful applications, the computational approach makes a series of further assumptions. Representations are static structures of discrete symbols. Cognitive operations are transformations from one static symbol structure to the next. These transformations are discrete, effectively instantaneous, and sequential. The mental computer is broken down into a number of modules responsible for different symbol-processing tasks. A module takes symbolic representations as inputs and computes symbolic representations as outputs.

This is indeed a popular formulation of the computational theory of mind originally defended by Putnam and Fodor, but I'm not sure I've seen it endorsed in so many incorrect details by a major Less Wrong author. For example my post Neuroscience of Human Motivation disagrees with the above description on several points.