Khaap, or Khap as it can also be spelled, is a word that can mean a group in general. Being specific, a Khaap is a group of villages, and here it's more or less used for Khaap Panchayats, groups made in villages to give their decisions, majorly on caste-issues.
These Khaap Panchayats are not the same as the Graam Panchayats, which are the officially elected and make the basic building block of Indian democratic system in villages.
The movie Khap takes into account the issue of honor killings based on the decisions of these Khaap Panchayats or Caste Panchayats, and hence the name.
[For more detail on Khaps, you can check this page on The Hindu, which even has some comments specifying how Khaps are Not involved in honor killings and explaining a lot more.]