#45-45-90" "# refers to the angles of the triangle.
The #color(blue)("sum of the angles is " 180°)#
There are #color(blue)("two equal angles")#, so this is an isosceles triangle.
It therefore also has #color(blue)(" two equal sides.")#
The third angle is #90°#. It is a #color(blue)("right-angled triangle")# therefore Pythagoras' Theorem can be used.
The #color(blue)("sides are in the ratio " 1 :1: sqrt2)#
It has #color(blue)("one line of symmetry")# - the perpendicular bisector of the base (the hypotenuse) passes through the vertex, (the #90°# angle).
It has #color(blue)("no rotational symmetry.")#