Meaning -
Seven are the bonds and bounds of the vedi (altar) of the mental yajna in meditation, thrice seven, i. e. , twentyone are the samidhas (fuel sticks) ordained. When the saints and sages enact the yajna within, they concentrate on the Purusha, the universal observer, whose presence they crystallise and fix upon in the soul. Note: Seven bounds and thrice seven samidhas is the mystique of yajna which has been explained in different ways. Swami Dayanand explains it thus: Seven bounds are the seven chhandas (metrical compositions) of the Veda-mantras such as Gayatri. Twenty-one samidhas are: Prakriti (the potential material cause of the universe), Mahat (actualised material cause), Ahankara (individualized identity of the universe), five subtle elements or Tanmatras, fivegross elements, five faculties of perception, and three qualities of the material universe which are Sattva, Rajas and Tamas (Thought/intelligence, energy and matter). In Rgvedadi-Bhashya-Bhumika, his Introduction to the Commentary on the Vedas, he explains the same thing in a different and detailed manner in the chapter on Creation. For an average reader, the simpler way to understand is to refer to the seven stages of creative evolution: Prakriti, Mahan, Ahankara, Tanmatra, Perception faculty, Volition faculty, and gross elements. These stages of evolution are the seven bounds of creative yajna performed by Purusha. These are also the Samidhas because each succeeding evolute arises from yajnic consumption of the previous one. And each one has three orders of quality, Sattva, Rajas and Tamas. That makes it thrice seven. This is the Sankhya order of evolution, and ‘Sankhya’ is the word from which comes the English word ‘Science’ through Latin in which it is written as ‘Scienkia’. Another way to realise the seven boundaries is to refer to the seven Vyahritis: Bhuh, Bhuvah, Svah, Maha, Janah, Tapah and Satyam which are the boundaries of each loka or region in ascending order.