To overload something is to impose a greater burden on it than it can bear, and an overload is such a burden.
An electrical overload, for instance, occurs when you put too many active electric devices on a given power source: if they draw more power than it can safely handle, a fuse is blown or a circuitbreaker tripped—or the circuit shorts and your house burns down.
As DamkerngT. suggests, an information overload occurs when a system has more information than it knows how to deal with: it spends all its time trying to parse and coordinate the information instead of acting on the information which is relevant.
By the same token, a guilt overload would be more sense of guilt than someone can handle. In the incredibly silly programming example from which your passage is drawn, the author has invited you to consider what ‘emotional states’ you want to program into a virtual pet rock so it will exhibit ‘behaviors’ which may induce the ‘owner’ to interact with the program. He suggests that you have it exhibit a sad face when it has been ‘ignored’ for some time, since this will not create such a guilt overload on the owner as would, for instance, ‘depression’:
The iRock cries every time you close the page, requiring the user to leave the brower open to keep the rock from having a breakdown.