What does Page Life Expectance say about the instance?

I have installed monitoring software on a few SQL Server instances in the environment. I am trying to find bottlenecks and fix some performance issues. I want to find out if some servers need more memory.

I am interested in one counter: page life expectancy. It looks different on every machine. Why does it change often on some instances and what does it mean?

Please take a look at the data from the last week gathered on a few different machines. What can you say about each instance?

Heavily used production instance (1):

Moderately used production istance (2)

Rarely used test instance (3)

Heavily used production instance (4)

Moderately used test instance (5)

Heavily used data warehouse (6)