After Thanos collected the six Infinity Stones, he used them to wipe out half of the population of the entire universe. After that, he destroyed the Stones, and was killed by Thor.
Five years later, the Avengers used Pym Particles to travel across timelines to collect the Stones. Once all six Infinity Stones were collected, Hulk used them to bring back to life the half of the universe's population that was wiped out.
The erasure and later revival of half of the life in the universe became known as the "Blip".
The Blip had major repercussions for everyone in the universe, as several planets had to adjust to half of all the universe's population coming back after five years, and the absence of several people had consequences, either good or bad, for those people's personal lives.
In a parallel world named Earth-838, the Blip was commencing when the Illuminati were able to kill Thanos and prevent the event from happening, resulting in Thanos being killed on Titan.
The victims of this event are also known as "The Vanished".
The term "Blip" originated in Spider-Man: Far From Home, used to describe half of the population disappearing due to Thanos' Snap, the five year time period as well as the resurrection five years later.
