pitRu yagyam means satisfying the manes (pitRus) daily. It is one of the pancha mahAyagyas.
Manusmriti 3.70 says:
पितृ यज्ञस्तु तर्पणं , pitru yagyas tu tarpaNam
pitru tarpanam means offering libations of water and/or sesame seeds to the pitRus daily.
A householder (gRuhastha) should peform pitRu yagyam.
There are many karmAs in which pitRu tarpaNas are offered. So pitRu yagyam cannot be defined as one karma. It would include all the tarpaNas that are performed by householders daily. Whereas,
श्राद्धं वा पितृयज्ञ: स्यात् पित्रोर्बलिरथापि वा।
shrAddhaM vaa pitRuyagya: syaat pitrorbalirathaapi vaa|
pitRu yagyam can be performed in any of the following 3 ways according to the practices of one’s own sampradAyam/sootram.
1. Offering of part of cooked food to manes as part of bali haraNa or
2. Offering libations of water by doing pitRu tarpaN as a part of dEva Rushi pitRu tarpaNam performed just after snAnam. A detailed tarpaNa is performed offering water to each of the many classes of gods, Rushis and the pitRus. tarpaNam is also performed as part of brahamayagyam.
Those who are unable to perform the detailed tarpaNam can do this brief devarshi pitRu tarpaNam of 2 verses advised by Shankha: (Ahnika prakAsha of Viramitrodaya-pg 145; dharma sindhu- pg 258). and offering water (jalAnjali) thrice.
आब्रह्मस्तम्ब पर्यन्तं देवर्षिपितृमानवा:।
तृप्यन्तु पितर: सर्वेमातृमातामहादय:॥१
Abrahmastamba paryantaM devarShipitRumAnavA:|
tRupyantu pitara: sarvemAtRu mAtAmahAdaya:||1
ateeta kula kOTeenaaM saptadveepa nivAsinaaM|
Abrahma bhuvanaallokaadi idam astu tilodakam||2
ஆப்ரஹ்மஸ்தம்ப பர்யந்தம் தேவர்ஷிபித்ருமானவா:|
த்ருப்யன்து பிதர: ஸர்வேமாத்ரு மாதாமஹாதய:||1
அதீத குல கோடீநாம் ஸப்தத்வீப நிவாஸிநாம்|
ஆப்ரஹ்ம புவநால்லோகாதி இதம் அஸ்து திலோதகம்||2
The meaning of the verses go like this:
‘May the gods, sages, pitRus, from Brahma up to a blade of grass, be satiated and also all the pitRus, the manes in the maternal line like the maternal grand-father and the elders, may this water mixed with sesame be for the crores of families of bygone ages residing in the seven dveepas from the world of Brahma downwards’.
3. Nitya shrAddham- Inviting a brAhmaNa for lunch daily to please the manes:
Manusmriti 3.82 &3.83
एकम् अप्याशयेद्विप्रं पितृ अर्थे पाञ्चयज्ञिके।
kuryaad aharaha: shraaddham annaadyena udakena vaa |
payo moola phalair vaa api pitRubhya: preetim Avahan ||3.82
ekam apyaashayed vipraM pitRu arthe paanchayagyikE|3.83
Let a householder invite daily a brAhamaNa for a shrAddh and please him with food and water or milk, fruits or that which please him. This would mean pitruyagyam in the panchamahAyagyam. (This nitya shrAddham is not as elaborate as the annual shrAddham. There is no hOma and no pinda pradhAnam. Only bhOjanam)