Time lines show what time - past, present or future - a particular tense covers.
Example :
Yesterday, I met Bob.
In the classroom
Time lines are one of many ways to talk about how verb tenses work. They are a visual representation, so they are useful for some learners with learning styles that respond to visual information. However, they can be confusing for others.
Further links:
https://www.teachingenglish.org.uk/article/timelines
https://www.teachingenglish.org.uk/article/jazzing-grammar
https://www.teachingenglish.org.uk/article/checking-understanding