You have to be good at many things if you hope to be great at one.
So with this expression a jack of all trades basically means a man of all jobs in other words it s someone that is competent at several different skills.
The headmaster of Charterhouse School, Martin Clifford, in a collection of notes on the poems of Dryden, circa 1677 wrote: Your Writings are like a Jack of all Trades Shop, they have Variety, but nothing of value.