The terms “flesh” and “spirit” are understood in connection with, and in contrast to, each other. That is, to appreciate what the Bible means by “flesh” you need to understand what it means by “spirit” – and vice versa. Therefore, when studying one of these terms, keep in mind the other.
For example, in Genesis 2:7 God made Adam by breathing life into the dust of the ground. That was a union of spirit (breath) and flesh (dust). What’s flesh is visible; what’s spirit is invisible.
Related post: The Holy Spirit.