A party line in politics means that people belong to one specific group with similar beliefs and goals, like being on the same team. It's like if you and your friends are in a club, and you all have to agree on certain rules or activities to be part of it.
In politics, parties with similar ideas join together to work together in government. These parties have their own goals and ideas for how to run the country. When it comes to voting on things in the government, party members usually stick to their party line and vote the same way, like a team would.
For example, if a party believes strongly in protecting the environment, they will work together to make policies that help the environment. They may also all vote in the same way to protect the environment, even if some of them don't like the specific details of the policy.
In summary, a party line is when people in politics with similar beliefs and ideas join together to work towards common goals, and they usually vote the same way on issues.