"Here" refers to the location(s) that the card occupies. For most cards, such as minions and artifacts, that means one location. For example, .
For auras, that usually means 4 locations, most commonly the surface of 4 sites. For example, . Another example is the use of "here" on , which includes the site surfaces but could also include void squares as well, if the aura occupied a void square.
When a site itself uses the word "here," that includes both the site's surface location and it's subsurface location. For example, protects minions in its subsurface as well as surface. When an effect cares about a card entering "here" in reference to a site, that means to enter any of the site's locations from outside of the site.
"Here" is analogous to "there."