What's the left blue line means?

[EasonWang01]

Sometimes it will appear sometimes not.

[johnfn]

It means you've modified that line since you last made a commit in version control.

[EasonWang01]

thanks but sometime I got green what's that means

[johnfn]

Green means you've added an entirely new line.

Oh and just in anticipation of your next question, red means that you've deleted some number of lines.

[EasonWang01]

Thanks for reply,found some article here. https://code.visualstudio.com/docs/editor/versioncontrol#_gutter-indicators