That’s the sim bar.

The number and bars tell you how fast the sim is, at a certain point in time.

Generally, anything above 1500 is excellent, above 1000 is very good, above 600 or 700 is good, and above 300 is ok. Below 100 is bad.

The sim’s performance depends on the type of sim (Class 5, 4, 3, or Homestead, in decreasing order of performance), as well as number of avies on the sim (more is bad).

Because your LULU gear is heavily-scripted, its performance will depend on the sim’s performance.

The number is only a rough guide, and it does not measure asset server issues, loading times, communication delay and failures, network connection and such, which also impacts your SL experience.