5301991f_and_The_Art_of_Breaking_Things__detailed_explanation
[signal] Biting lip [part of body] mouth [possible meaning(s)] tension [detailed explanation] 
One of many signals suggesting tension or stress, which can be due to high concentration, but more likely to be anxiousness.
“I pretty much try to stay in a constant state of confusion just because of the expression it leaves on my face.” – Johnny Depp | Zen and The Art of Breaking Things
detailed explanation