What does Page Life Expectancy 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)

EDIT: Im adding the output of SELECT @@VERSION for all of these servers:

Instance 1: Microsoft SQL Server 2008 R2 (SP1) - 10.50.2500.0 (X64) 
Jun 17 2011 00:54:03 Copyright (c) Microsoft Corporation
 Standard Edition (64-bit) on Windows NT 6.1 <X64> (Build 7601: Service Pack 1) (Hypervisor)


Instance 2: Microsoft SQL Server 2012 (SP1) - 11.0.3000.0 (X64) 
Oct 19 2012 13:38:57 
Copyright (c) Microsoft Corporation
 Standard Edition (64-bit) on Windows NT 6.1 <X64> (Build 7601: Service Pack 1) (Hypervisor)


Instance 3: Microsoft SQL Server 2012 - 11.0.5058.0 (X64) 
May 14 2014 18:34:29 
    Copyright (c) Microsoft Corporation
 Developer Edition (64-bit) on Windows NT 6.1 <X64> (Build 7601: Service Pack 1) (Hypervisor)

Instance 4: Microsoft SQL Server 2008 R2 (SP2) - 10.50.4000.0 (X64) Jun 28 2012 08:36:30 
Copyright (c) Microsoft Corporation
 Standard Edition (64-bit) on Windows NT 6.1 <X64> (Build 7601: Service Pack 1) (Hypervisor)


Instance 5: Microsoft SQL Server 2012 - 11.0.5058.0 (X64) 
May 14 2014 18:34:29 
Copyright (c) Microsoft Corporation
 Developer Edition (64-bit) on Windows NT 6.1 <X64> (Build 7601: Service Pack 1) (Hypervisor)

Instance 6: Microsoft SQL Server 2008 R2 (RTM) - 10.50.1600.1 (X64) 
Apr 2 2010 15:48:46 
Copyright (c) Microsoft Corporation
 Standard Edition (64-bit) on Windows NT 6.1 <X64> (Build 7601: Service Pack 1) (Hypervisor)

I also ran the following query on the machines:

SELECT DISTINCT memory_node_id
FROM sys.dm_os_memory_clerks

and it returned 2 or 3 rows for each server:

Instance 1: 0; 64; 1
Instance 2: 0; 64
Instance 3: 0; 64
Instance 4: 0; 64
Instance 5: 0; 64
Instance 6: 0; 64; 1

What does it mean? Do these servers run NUMA?