When a person's conscience is in the act of no longer subjecting itself to reality.
It can be anywhere from decreased ability to function due to lack of sleep, to the process of going batshit insane.

Most commonly it's seen used for people under the effects of Hallucinogens (i.e. psychedelics, dissociatives, and deliriants)
Another use is if they are panicking due to some sort of shock, paranoia, or fear.
Generally anything that effects a persons conscience to a degree where it can't function at it's normal performance can cause them to start 'tripping out'.

It is not reserved for use on or by someone under the influence of drugs.

When you eat dirt for the first time and leave your family to build a shack in the woods

The act of immersing oneself in the sensation of one or more stimuli. Tripping out is when someone focuses strongly on a sensation, feeling, or sensory input. Examples of behaviour that may result in tripping out include listening to music (along with dancing to said music), and rubbing one's hand on something soft. The mental state of someone while tripping out is consumed by the sensation on which they are tripping out. For example, when tripping out to music while dancing, the mind becomes fixated purely on the sensations of the music and the dancing in the present, such that awareness of other stimuli becomes ignored. Strong tripping out may look strange to outsiders.