“Look At” is the name given to a beacon, cross hairs, which indicate where an avatar's camera is focused. Where an avatars camera is focused is not necessarily where they are looking, it is only where the focus point is.
If you want to know what the crosshair colors mean, please refer to this SL wiki page.
To see where other avatars have focused their camera:
Please note that others can disable their Look At from being seen (see below), so enabling this doesn't guarantee that you will see everyone's camera focus.
Note also that this doesn't show you your own Look At beacon, but see below.
In order to prevent others from seeing where your camera is focused:
Enable Go to Preferences → Privacy → LookAt → Don't show any look at targets at all, not even to myself - disable.
Refer to this page.