Binocular fusion (simply "fusion") refers to the brain's ability to gather information received from each eye separately and form a single, unified image. If the eyes are not precisely aligned, a patient may experience blur, double vision, or discomfort.
The brain has an interesting adaptation specific to double vision. In young patients, the visual system is incredibly versatile - the fancy term for this "plastic" as in "neuroplasticity". What this means is that the visual system can adapt to the confusion of double vision by ignoring or suppressing the second image. This is a subconscious task and is referred to as (cortical) suppression. Double vision often occurs from an eye that is not aligned. It may be turned inward (esotropia), outward (exotropia), or up (hypertrophy) or down (hypotropia). That eye may then develop poorer clarity of vision or visual acuity as well - this is known as amblyopia or lazy eye.
Some patients may have difficulty with fusion only at certain distances. For example, a patient may have normal fusion at distance but struggle with fusion up close with reading. The result is blurry our doubled images when trying to do near work. This example is what happens to patients with convergence insufficiency.