Everything material is a mental concoction because it is sometimes visible and sometimes not. At night when we dream of tigers and snakes, they are not actually present, but we are afraid because we are affected by what we envision in our dreams. Everything material is like a dream because it actually has no permanent existence.
Śrīla Viśvanātha Cakravartī Ṭhākura writes as follows in his commentary: arthena vyāghra-sarpādinā vinaiva dṛśyamānāḥ svapnādi-bhaṅge sati na dṛśyante tad evaṁ dārādayo 'vāstava-vastu-bhūtāḥ svapnādayo 'vastu-bhūtāś ca sarve manobhavāḥ mano-vāsanā janyatvān manobhavāḥ. At night one dreams of tigers and snakes, and while dreaming he actually sees them, but as soon as the dream is broken they no longer exist. Similarly, the material world is a creation of our mental concoctions. We have come to this material world to enjoy material resources, and by mental concoction we discover many, many objects of enjoyment because our minds are absorbed in material things. This is why we receive various bodies. According to our mental concoctions we work in various ways, desiring various achievements, and by nature and the order of the Supreme Personality of Godhead (karmaṇā-daiva-netreṇa (SB 3.31.1)) we get the advantages we desire. Thus we become more and more involved with material concoctions. This is the reason for our suffering in the material world. By one kind of activity we create another, and they are all products of our mental concoctions.