"Perimeter" means the distance all around the figure.
So it's the sum of the lengths of all the sides.
Here are a few possibilities with Perimeter = 6 units:
-- a square, with every side 1.5 units long
-- a triangle, with every side 2 units long
-- a pentagon, with every side 1.2 units long.
-- a hexagon, with every side 1 unit long.
The sides don't all need to be the same length;
they just have to all add up to 6 units.
-- another triangle, with sides 1 unit, 2 units, and 3 units long.