It means someone that can do a little bit of everything. He can perform many different jobs, but is the master of none. For example, a man who can fix a leaky faucet, repair a broken lamp, install a furnace, and replace the carburator on a car, but yet doesn't have the required education or expertise to be certified as a plumber or a mechanic, would be a jack-of-all-trades.
"Jack-of-all-trades, master of none":
Someone who is able to do a lot of things, but none of them with the skill one woulsd expect from a true professional...
jack-of-all-trades 
He can perform many different jobs, but is the master of none. For example, a man who can fix a leaky faucet, repair a broken lamp, install a furnace, and replace the carburator on a car, but yet doesn't have the required education or expertise to be certified as a plumber or a mechanic, would be a jack-of-all-trades.
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