That means that the task being run will be cancelled if it is involved in a deadlock. It doesn't mean that a deadlock will occur. -10 is the lowest priority possible, so, by default, every other task will be considered more important.
By default, deadlock priority is 0. If two tasks get into a deadlock, SQL uses its own stats -- which comes down to SQL determining "which task being cancelled would require less work" -- to determine which one to cancel.
But, if one task has a lower priority than the other one(s), SQL will cancel that task, no matter what the stats are for each query.
SQL DBA,SQL Server MVP(07, 08, 09) Prosecutor James Blackburn, in closing argument in the Fatal Vision murders trial: "If in the future, you should cry a tear, cry one for them [the murder victims]. If in the future, you should say a prayer, say one for them. And if in the future, you should light a candle, light one for them."