The assembled Khals discuss and decide which lands to raid and which tribes to enslave. It is unclear how frequently each gathering occurs.
According to the High Priestess of the Dosh Khaleen, a khalar vezhven is imminent, and one of the things the khals will decide is the fate of Daenerys Targaryen, who violated Dothraki tradition by venturing out into the world instead of joining the dosh khaleen immediately. During this gathering, the khals deemed Daenerys unworthy of becoming a dosh khaleen and in response, she killed every khal in attendance as well as numerous bloodriders by knocking over every lit brazier in the building, setting it ablaze. When she emerged unscathed, every surviving Dothraki outside swore their allegiance to her, restoring her title as Khaleesi, albeit with a much larger force of 100,000 at her command.
In the books
In the A Song of Ice and Fire novels, the specific khalar vezhven seen in the TV series has not yet occurred, because Daenerys is only captured by the Dothraki in the last chapter of the most recent novel (corresponding to the Season 5 finale).
The overall custom of an assembly called a khalar vezhven has actually not been described in the novels - yet.
The prior novels did vaguely mention that Vaes Dothrak is large enough to hold all of the Dothraki khalasars when they periodically all return to the city, apparently for different cultural gatherings or religious ceremonies, etc.
"Vezhven" means "Great", "Khalar" might mean "gathering of Khals".
The concept of the khalar vezhven is similar to the real-world kurultai, where the Mongol khans gathered to discuss important issues, including future invasions and the election of a new Great Khan.