"Indrek Aavisto" wrote

While flying on a 777 I noticed a series of what appeared to be small
fences close to the leading edge of the wing. Can anyone tell me their
purpose?

Do a Google for Vortex Generators.

From Wikipedia:
Vortex generators are likely to be found on the external surfaces of
vehicles where flow separation is a potential problem because vortex
generators delay flow separation. [3] On aircraft they are installed on the
leading edge of a wing in order to maintain steady airflow over the control
surfaces at the rear of the wing.[2] They are typically rectangular or
triangular, tall enough to protrude above the boundary layer, and run in
spanwise lines near the thickest part of the wing.[1] They can be seen on
the wings and vertical tails of many airliners. Vortex generators are
positioned in such a way that they have an angle of attack with respect to
the local airflow.

Bob Moore