This is the IP address of one specific router that a tunnel's explicit path is required to traverse, and the description notes it becomes immutable once the hop entry is created -- you must delete and recreate the row to change it. An admin uses the ordered set of these hop addresses within a path to verify the LSP is actually being forced through the intended sequence of routers rather than an IGP-computed shortcut. For example, a path with hops 172.16.1.1, 172.16.2.1, 172.16.3.1 in that order tells the admin the LSP must transit exactly those three routers, in that sequence, regardless of what the IGP would otherwise prefer.