What is a whip in a political party?

A whip is the term for an individual who is a member of a political party in parliament (an MP). The leadership of the party relies on the 'whip' to try to ensure that all the members of the party are voting along party lines (voting in agreement with the party leadership and manifesto). Their most important role is to implement their parties ideology throughout the party in parliament. This primarily focuses on promoting the policies to the back benches (MP's who do not have an important role within their party) who are more likely to vote in line with their constituents and/or their personal interests and ideals.
An example of a Labour whip at the moment would be the Rt. Hon. Nick Brown, who has been the MP for Newcastle East for decades and has held important roles within the Labour party such as being the chair of the LGBTQ+ committee along with over responsibilities. His commitment to the Labour party over the years means that the Labour party leadership has trusted him to enforce the party line and be a whip.