loose line and tight line

Thanks Nigel, it's a privilege to have you around.

Please do not get obssesed with those yellow highligted lines, but the most intense or obvious. There is not always possible to eliminate them all, and it may become a tighted or uneven and unnatural text fitting. Use your eyes on printed text of some pages, before taking a decision.

You may go up to some settings like those displayed here. And pay attention too, to Hyphenation values. It's all a matter of balance.

Nigel French videos on Typography in Lynda.com, are your best resource for learning about this subjects. Highly recommended to everyone.

Federico's answer is correct. 

Just to add a bit more information…

Turning on H&J Violations will show where InDesign can't honour the justification settings. The more vivid the yellow, the worse the problem. This isn't quite the same as loose lines and tight lines since it's possible to have really lax justification settings where few violations will be highlighted, or really strict justification setting where virtually everything will be highlighted. To put it another way, changing the justification settings to remove the yellow highlighting is not a cast iron guarantee that you won't have loose or tight lines. You need to let your eyes be the final arbiter.