I will assume since this is to commemorate Summorum Pontificum this is in the Extraordinary Form of the Roman Rite, and I will assume you have attended a EF Mass.
A Pontifical High Mass is the Paradigm of the Roman Rite, it is the Ideal way of celebrating the Liturgy.
There are two ways of celebrating the Pontifical High Mass, the difference between the two is where the Bishop is sitting. There is a Pontifical High Mass at the Throne. Or at the Falstool.
If this is the Bishop of your Diocese, at the Throne means from the Cathedra, which is the seat of the Bishop at the… Cathedral.
The Mass will be mainly the same as a regular Solemn High Mass, except:
-There is an extra 7th Candle at the Altar.
-The Bishop bows instead of genuflecting , even with a Tabernacle.
-He remains at the Throne for the Mass of the Catechumens(Liturgy of the Word), he reads the Introit there, does the Kyrie there, intones the Gloria, prays the Collect, gives the blessing for the Gospel, etc.
- He doesn’t process in with the Maniple, he only puts it on during the Prayers at the Foot of the Altar.
-In addition to the Deacon and Subdeacon of the Mass, there are two Deacons of the Throne, who sit with the Bishop and assist him there.
- There are four special servers, usually with copes, who carry the special pontifical items, the Miter, Crozier, Book, and Candle Bearer.
-there is a second and third Lavabo prior to going to the Altar for the Offertory, and after the Ablutions.
-He greets with “Pax Vobis” instead of “Dominus Vobiscum”.
- at the Final Blessing he makes thre crosses instead of one
- He wears a light version of the Tunicle and Dalmatic underneath the Chasuble.
A Pontifical Mass at the Falstool is pretty much the same except:
- He sits on a stool on the right side of the Sanctuary facing the people, isntead of the throne.
-There are no Deacons of the Throne.
-No 7th Candle on the Altar
- And usually, he doesn’t carry the Crozier with him. Which also means no Crozier-bearer.
What this means for you and the laity, nothing! Just do what you usually do at Mass, if it is you attend the EF form.