Jobbing in the stock market is an ultra-short term trading of securities with the intent of generating small profit by analyzing the job spread.
By the ultra short term, I mean the time frame as low as a minute and normally such trades are squared off within a minute.
There are buy orders with bid price and sell order with ask price. The strike price is when the bid price and ask price match and order is executed. The gap between the ask and the bid price at any given point of time is known as job spread.
Traders who do jobbing trades are termed as jobbers and they trade continuously throughout the day to bag small profits in each trade.
Jobbing generates a lot of trades, which also means higher brokerage. So these trades are often undertaken on a profit-sharing basis with the broker instead of fixed brokerage or are executed directly on the exchanges without a broker.
Note: Exchanges try to avoid jobbing spreads and you an read about their system to avoid jobbing here.
Traders can even adopt software to identify jobbing opportunities and perform these jobbing trades on behalf of the investors.
Jobbing needs a high amount of concentration and jobber needs to be very quick in placing orders to be executed and is only recommended at a very young age.
Jobbing is not something that anybody can do and before you can even think about jobbing, you have to be trained for spotting job spreads in the first place, let alone executing the jobbing trades.
Jobbing is completely speculative in nature and is best avoided for retail investors.