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Panic attacks lead to constant fear of another one and hence what is called panic disorder develops. You are scared to leave the house, scared to go into a particular room of the house even, for fear of having another one. Also, the physical effects of an attack can leave you unable to function properly for hours, so that can have an impact as well.

If you read my answer to your earlier question you will understand your text more. Again, here is an excerpt from anxietypanic.com that relates to this question:

After a few months of panic, about 10% of people become housebound and unable to leave home alone. After a few years, about 30% of panic sufferers have a loss of job, pay or job responsibilities. Some 17% are at risk for alcoholism and about 40% risk a chronic depression as life opportunities are cut off. A majority have marital problems and much reduced travel and social life. The economic cost has been estimated at about $2600/year in misdirected treatment, and about $12,000 a year in lowered job earnings. For most, panic closes life off like a prison.

To "function" means to "be functional". In other words he is able to perform his tasks, be they activities of daily living or, perhaps even more complex and difficult activities.

According to the text, the implication is that there is, in fact, no activity that is prevented by the panic attacks. He deals with the anxiety through behavioural changes, which could be quite complex or something quite simple, such as crossing himself before travelling.