a mirror plane
A mirror plane does exactly what a bathroom mirror does: it reflects one half of an object into the other. Hold your hand up to a mirror and the reflection completes a whole; a shape has a mirror plane if some flat slice through it acts as that mirror, with everything on one side matched by an identical copy on the other. A human face is roughly mirror-symmetric left-to-right; a plain isosceles triangle has one mirror line.
In symbols the mirror plane is m in the international (Hermann-Mauguin) notation and sigma in Schoenflies. Reflection reverses handedness: your right hand reflects into a left hand. A point at coordinates (x, y, z) reflected in the plane perpendicular to x lands at (-x, y, z). Schoenflies distinguishes flavours by where the plane sits relative to the main axis: sigma-v (vertical, containing the axis), sigma-h (horizontal, perpendicular to it), sigma-d (dihedral, bisecting two 2-fold axes).
A mirror plane is an improper operation, one that cannot be produced by physically turning the object, only by 'flipping through', and its presence forbids a crystal from being chiral. Crystals that must show handedness, such as quartz or table sugar, contain no mirror plane at all. Reading the m's in a point-group symbol immediately tells you which mirror planes a crystal has.
Point group mm2 has two mirror planes at right angles; their line of intersection is automatically a 2-fold axis, so combine two perpendicular mirrors and a 2-fold rotation appears for free.
A mirror plane reflects one half of the structure onto the other and reverses handedness.
A mirror is the rotoinversion axis 2-bar in disguise: reflecting in a plane is identical to a 2-fold rotoinversion about the axis perpendicular to it. Handy when translating between notations.