What is something that can be experienced but cannot be described ?

"Description" is fungible; one's love is describable, in part; one's experience of a sunset or a sunrise, a rose or a smile,  is describable, in part.  Per Godel, it is not possible to "completely describe" any sufficiently complex set, i.e., there are some descriptions that are indeterminate ("incompletely explicative") re their experienced verity or reality.

The cognitive neurostructure of a Zen realization is an example of clear calm in neural activity, followed by a novel ideation.  The ideational formation is primary, with description of the idea per awareness of its structure, topos, and/or domain; however, the initial formation is pre-cognitive, atopic, ineffable.  References:  "Zen and the Brain" and "Ineffability:  The Failure of Words in Philosophy and Religion."

Try describing the colour blue to a blind person who has never seen the colour of anything.

In a word ineffable 

Death. Nobody has ever come back to describe what it's like. Near-death experiences don't count. 

Welcome to the term "qualia."

The texture of crunchy peanut butter.

The taste of a particularly tart pinot grigio.

The color blue.

The way I feel right before a migraine.

The enjoyment of the day's first cigarette.

How thrash metal makes me feel.

A closet homosexual encounter.