requerimiento notarial is a formal communication carried out by a notary public.
For example, I’m the developers lawyer and the deadline for completion is close at hand and I see you are unwilling to complete.
I send you a requerimiento notarial compelling you to complete in a certain date, at a time, in a notary. Should you fail to appear, legally I could pull out of the contract and keep -normally- 50% of all the money you’ve paid up to date (depends on the draconian contract stipulations).
A requerimiento notarial is a certified Public Notaries communication which will be used at a later date in a court procedure as unrebuttable proof.