A user story is the account/opinion of a person that uses the software provided or is personally responsible for deciding what updates this software needs. It is taken by the product owner, usually by talking to the people in question, sometimes through surveys, and then compiled into a specific format that shows the importance, effort required, and details about the changes and who asked for them.
In order to implement a user story, the product owner, sometimes with the help of the team, develops a product backlog, this is a list of user story sorted from most important to least important, taking into account the effort level required for each.
User stories allow teams to be extremely agile in the development process, by giving them the ability to stop working on a user story in order to fulfill a more important one, like an urgent update.
As a software developer, I approach writing software as a team effort. There are plenty of agile processes that help making that possible, like daily scrum meetings, sprint planning, sprint review and retrospective. These are the basic processes that every scrum-agile team is using, but there are some other processes that help that not everyone uses, like Jira's planning system.
In order to be a good team member in a scrum-agile team, you have to be respectful of others, communicate well, help others when needed, and voice your opinion where you feel that is fit to do so. Other aspects of being a good team member include the ability to self-manage, and stay on top of your work without needing anyone to tell you what you need to do.