The word 'ismah literally means “protection”. In Islamic terminology, it means “infallibility”. Infallibility is defined as “a spiritual grace (lutf) of Allah to a person which enables him to abstain from sins by his own free will.”1
A person who has been given that grace of God is called a ma'sum , infallible, sinless.
This power of 'ismah does not make the ma'sum person incapable of committing sins. A ma'sum refrains from sins and mistakes by his power and will. If it were otherwise, then there would be no merit in being ma'sum! A ma'sum is able to abstain from sins because of (a) highest level of righteousness, and (b) ever-present consciousness and love for God, and (c) certain knowledge about the consequences of committing sins.
We come across some ordinary people who are very upright in their character and would not even dream of committing certain sins or crimes. The strength of their character makes them relatively an infallible person.
For example, it is within the power of any person to go naked on the streets. But would a person who was brought up as a good Muslim, ever think of doing so? No, because it is far below his dignity to behave in this way. It is not impossible for him to do so, but he will never even imagine of doing so. Why? Because he has been taught and brought up with the idea that such behaviour will tarnish his honour and is beneath the dignity of a civilized human being.
Similarly, though a ma'sum has the ability to commit sin, he never even imagines of committing a sin because (a) the love for God in his heart leaves no room for displeasing Him by committing sins, and (b) he is full aware of the consequences of committing sins.