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It could mean that he simply wants more money. By opting out with two years left on his current deal, LeBron can sign a new five-year contract with Miami. He can also re-sign under the new league-wide maximum salary, which is up to $22.1 million after the salary cap unexpectedly went up. That'd be a ~$1.5 million raise from the 2014-15 salary figure under LeBron's old contract.
It could also mean that he wants less money. There's also sorts of speculation that the Heat Big 3 will rework their contracts in order to create enough cap space to bring in another star and/or role players. This seems unlikely, but opting out be the first step in the process.
And finally, it could also mean that he wants to leave Miami.
If he does want to go elsewhere, he'd be leaving a ton of money on the table. Under NBA rules, LeBron can get a five-year deal with 7.5% annual raises if he re-signs with Miami. If he signs with another team, he can only get a four-year deal with 4.5% annual raises.
The difference is $32.7 million:
LeBron left money on the table when he ditched Cleveland and signed with Miami in 2010.
He'd have to do it again this time around if he wanted to leave the Heat.