Once an admin assigns a task to you, it shows up in the Incoming tasks widget. You will see the task title, description and target languages along with the progress statistics.
Click a language, and it will open the editor with all the items from this task, or click the number of words to do to open only the uncompleted keys.
Words to do counters are updated as the translation process progresses. Depending on the configuration of the task set by the creator, you may need to either manually mark a language as done, or it will be marked as done automatically.
Once all the languages are marked as done, the project admins will receive a notification and may completely close the task.