The prefix dextro comes from the Latin word dexter

The prefix dextro comes from the Latin word dexter. It means "on the right side" or "to the right".

The prefix levo comes from the Latin word laevo. It means "on the left side" or "to the left".

We see these terms when we talk about chiral molecules rotating the plane of polarized light.

If the plane of polarized light is rotated clockwise as it approaches the observer (to the right if you are steering a car), the molecule is dextrorotatory (d).

If the plane of polarized light is rotated counterclockwise (to the left), the molecule is levorotatory (l).

We also see the terms dextro and levo when talking about sugars.

For example, glucose exists as D-glucose and L-glucose.

Here, the D and L do not refer to the direction of rotation of polarized light. Instead, they refer to the Fischer projections.

If the OH in the highest-numbered chiral carbon (closest to the bottom) is on the right, we have a D-sugar. If the OH is on the left, we have an L-sugar.