Jesus came into the world and put sandals on the ground in order to save it. In our world today, the phrase "boots on the ground" describes a higher level of investment and commitment. It means real people are going into dangerous situations and are willing to sacrifice themselves for a purpose.
Boots on the ground is a military concept and not appropriate to apply Jesus but, for us today, it does conjure up the concepts of what he did when the Word became human and dwelt among us.