The so-called practice is actually the source of knowledge and the driving force for the development of knowledge. Practice is the only criterion to test the truth of cognition, and practice is the purpose and destination of cognition. People subjectively think that it is because people see this, but sometimes these are not necessarily correct. For example, through various phenomena in daily life, we have developed mathematics, and all mathematical principles come from our practical activities. This also reflects that practice is the source of knowledge, and then with the development of society, we need more and more complex practical activities to promote the continuous development of mathematics. It is our practical activities that promote the development of mathematics, but our study of mathematics will ultimately serve our daily activities. Whether these mathematical principles are correct or not depends on whether we are satisfied with our practical activities.
My understanding of this sentence is that practice is the only criterion for testing truth. The so-called truth is a philosophical category, which marks the consistency of subjectivity and objectivity. The truth of cognition depends on whether the subjective and objective are consistent. These can only be based on practice. Since it serves practice, it must be tested in practice. For example, in the third mathematical crisis, about Russell's paradox, "If R is a set that does not contain itself, then the R package does not contain R itself?" If we don't explain this problem, there will always be problems in our basic collection of mathematics, but we are still studying mathematics now because mathematics is really useful to us. Our practical activities need it, so mathematics has always existed.
All the above are my opinions, and have nothing to do with the professor!