orzeczenie normallly means 'verb', but in your context the correct technical translation would be "predicator"
This is a definition of the modal verb, isn't it?
It says "part of the predicator (the verbal part of a sentence) which represent an expression of probability related to the verb in the infinitive" (this is a loose translation, a more exact one would be "which imparts a shade of probability - THIS SOUNDS VERY CONFUSING ALSO IN POLISH. 'a degree of probability' would perhaps sound better - to the verb in the infinitive"
must have come early = most probably came early
thus
'must' expresses a high degree of probability
Hope it's clearer now.