Student or Learner
From the semantic point of view, modal auxiliaries are often specialized
towards the expression of certain speech acts; eg giving advice (ought to,
should), making promises or threats (will), giving orders (must, can), etc.
(A Comprehensive Grammar of the English Language)
Have I understood the part in bold?
Certain speech acts can be expressed through modal auxiliaries.
Yes, though I'd reverse the order:
Modal auxiliaries can express certain speech acts.