It's line of sight only. Think about it like a flashlight. If you have a flashlight (w/power) up on top of a skyscraper roof or a mountainside it can be seen at very long distances. At street level it goes till the next small rise in the ground.
Pretty much everything from VHF and up (all of wifi) is line of sight only. Not just the mm-wave stuff. Cell phone networks only work because the telcos pay big money to get their transceivers power/data up on towers high above the terrain.
Store and forward meshes assume the nodes will eventually see each other but down at 5ft above the ground they often won't.