Line of Sight.
LoS pull means a tank engage enemy with range attack and hides behind Line of Sight (los) forcing them to come in his desired place, usualy by the corner they are grouped which makes it easy to maintain aoe threat, and are easier to get AOE beaten without losing threat by tank.
I rarely use it on my tank if I pve (unless a friendly healer asks, or pug healer asks politely) maybe with one exception in Cademinu, not a hard pull but spread and very very annoying, but I don't mind tank doing it, it's his choice (but I preffer them not to, and never asked any tank to do so).
This reminded me of a horrible run I had with my Shadow when I first started Tanking on Shadowlands. I was in horrible gear (PvP Tank/DPS gear and maybe some Columi gear). I had found out the meaning of LoS on a previous run in Maelstrom Prison. The tank at the time said he would LoS the final boss to another room where the adds would not reach us (before they fixed it). So when I do the same run as a Tank a few days later I figured I'd do the same thing...
However the DPS decided to follow me and engaged the boss right there despite me telling him to keep running. So now I'm trying to get aggro and AOE taunt and get the bosses attention and the turrets for the moment until the droids come out...I'm still trying to "LoS" to the other room but the DPS is not having it and keeps grabbing aggro from me. I notice I'm almost full health and both DPS are low and the healer is telling me I did not taunt at the right time in chat lol. I get booted and scarred from the experience and only PvP for the next few months while leveling a Guardian. I'm glad LoS now is more common knowledge though and when I say it people stay put for the most part. (I still have DPS who think I need to be followed instead of staying put.