When there is a misalignment of the visual axes that, in general, remains hidden and only appears when performing certain maneuvers such as covering an eye. This is so because phoria is compensated thanks to complementary eye reflexes, which are called fusion reserves, which are responsible for keeping the eyes straight, without deviations, despite the presence of phoria.
Treatment: First you must distinguish between phoria and strabismus. The main difference is that, in cases of strabismus, the deviation of the visual axes is manifest, appearing to a greater or lesser degree according to each specific case. Forias only have orthoptic exercises as treatment.