Bg 4.28

dravya-yajñās tapo-yajñā
yoga-yajñās tathāpare
svādhyāya-jñāna-yajñāś ca
yatayaḥ saṁśita-vratāḥ

Synonyms: 

dravya-yajñāḥ — sacrificing one’s possessions; tapaḥ-yajñāḥ — sacrifice in austerities; yoga-yajñāḥ — sacrifice in eightfold mysticism; tathā — thus; apare — others; svādhyāya — sacrifice in the study of the Vedas; jñāna-yajñāḥ— sacrifice in advancement of transcendental knowledge; ca — also; yatayaḥ — enlightened persons; saṁśita-vratāḥ — taken to strict vows.

Translation: 

Having accepted strict vows, some become enlightened by sacrificing their possessions, and others by performing severe austerities, by practicing the yoga of eightfold mysticism, or by studying the Vedas to advance in transcendental knowledge.

BG 4.28

Shri Krishna will describe different kinds of Yajna in this verse.

dravya-yajñās – is possession, wealth, tapo-yajñā – some people sacrifice in austerities, yoga-yajñās – some people perform astanga yoga, tathāpare – means some others, svādhyāya-jñāna-yajñāś ca – some people seriously studies so they sacrifice their intelligence, in this way yatayaḥ — enlightened persons, saṁśita-vratāḥ - saṁśita means strict, so they take strict vows and act accordingly.

When they are acting this way, what are they actually doing? They are trying to connect to absolute truth but acting on what is available to them what is accessible to them for a higher cause. Srila Prabhupad gives example of these

Srila Prabhupad’s purport: These sacrifices may be fitted into various divisions. There are persons who are sacrificing their possessions in the form of various kinds of charities. In India, the rich mercantile community or princely orders open various kinds of charitable institutions like dharma-śālā, anna-kṣetra, atithi-śālā, anāthālaya and vidyā-pīṭha. In other countries, too, there are many hospitals, old age homes and similar charitable foundations meant for distributing food, education and medical treatment free to the poor. All these charitable activities are called dravyamaya-yajña.

Srila Prabhupad doesn’t talk about this in any negative way, he is not saying this is all mundane issue don’t do it we should only chant Hare Krishna. So Krishna is telling this is one way of doing sacrifice…someone may say that they are doing this just for name and fame but at least they are not seeking name and fame by going to parties or trying to have big name by some low ways, even if they are looking for name and fame they are doing so by the way it benefits others. This indicates consciousness situated in higher mode

tapo-yajñā – Srila Prabhpad talks about cāturmāsya, cāturmāsya can become an intricate ritual, Srila Prabhupad didn’t go into too much of technical details.

candrāyaṇa -> is again a purifying process

Srila Prabhupad’s Purport: For example, under the cāturmāsya vow the candidate does not shave for four months during the year (July to October), he does not eat certain foods, does not eat twice in a day or does not leave home. Such sacrifice of the comforts of life is called tapomaya-yajña.

Minimize the need of the body and focuses more and more on the spiritual needs. There are two ways to not care for body, pertaining to our hair one is to neglect it to grow and other is to completely shave it off. In kalyuga it is said laulyam Kesh dharanam -> they will keep hair (beard and mustache in the same line) in different styles. So normally Vaishanavas shave off but in some cases some Vaishanavas may let it grow without bothering about it. It is austerity as one is not bothered about one’s look.

yoga-yajñās tathāpare
svādhyāya-jñāna-yajñāś ca

Srila Prabhupad’s purport: All these yogīs are faithfully engaged in different types of sacrifice and are seeking a higher status of life

So all these sacrifices are meant for elevation that Shri Krishna will talk about in one of the later verses.