Most likely current year salary converted to signing bonus

Signing bonuses are spread over the duration of the contract for cap purposes vs counting in the year they are given.

So say he was going to make $10mil this year, they sign him to an extension and convert $9mil to signing bonus and leave $1mil as salary this year (plus whatever he gets for the next two years). This year's cap hit is salary of $1mil + 1/3 of signing bonus, $3mil = $4mil. That's $6mil less than the $10mil he would have been against the cap and he still makes $10mil this year.

The counter is he is cut prior to end of bonus (or traded without so.e weird contract adjustment) then the remaining signing bonus and any other guaranteed money hits all in that year.