* Left and Right: These are adjectives describing direction.
* Straight: This is an adjective describing a lack of curves.
Therefore, "left and right straight" is a phrase composed of two adjectives. It describes a direction that is both left and right, and also straight.
It's important to note that this phrase is somewhat unusual because "left and right" are often used as adverbs (as in "Look left and right"). In this case, however, they are functioning as adjectives to modify the noun that would follow, such as "path" or "line."