When someone feels hopeless or does not know what to do about something. So it is used in replace to "well" sometimes in a conversation. A combination of "help" and "well".

the thing that you don’t have when you’re phones percentage is over 20

To be used in place of 'well', when one feels there is no more to say. This word was originally brought to the peoples' attention in 1994, from Jim Carrey in the cult classic, Dumb & Dumber.
This is simply one of the many words and phrases that continue to be used in teen conversation today.

welp is a word that can be used in any conversation to convey helplessness, distaste, and apathy to the subject. It can also be used as a way to end conversations when your tired of the other persons bullshit.

An interjection used when discussing an unfortunate circumstance without expressing anger. "Welp" is often used when exaggerating or seeking pity, sarcastically, or in attempts to conceal frustration. The 'P' sound at the end is usually muted, and simply serves as a consonant sound added onto the commonly known word "well."

an idiotic form of well.

It's a resigned declaration of existential helplessness. A recognition, if you will, of your insignificance in the face of destiny's vicissitudes. "Wel'p" is the apotheosis of the attitude Tilders (natives of thetldr.com) have been taking toward life since early adolescence, the quintessence of the emotion that's bound all neckbeards together through the ages. It is our curse, our blessing, our agony, and our ecstasy. It is wel'p, and it is what allows us to exalt paradoxically in the depths of our shared despair.