What is Ticket
A written unit of work in a tracking tool: one bug to fix or one feature to build, assigned to a developer.
In plain terms
Tickets live in tools like Jira, Linear, or GitHub Issues, and they move through states: open, in progress, in review, done. A ticket is the unit developers pick up, estimate, and complete. On GitHub, the equivalent object is called an issue.
What it tells you about a developer
Developers who write clear tickets and bug reports can specify a problem precisely before solving it, which is rarer than it sounds. Public issue activity in open source repositories is verifiable evidence of the skill.