The addition operator may be used to add numeric values:

2.71828 + 3.14159 

It may also be used to concatenate strings:

'hello'  + ' ' + 'world' 

If an attempt is made to concatenate a string with a number, the number will be converted into a string:

'hello number ' + 1 

This style of operation will fail if the number is used first. PowerShell expects the entire expression to be numeric if that is how it begins:

PS> 1 + ' is the number I like to use'
Cannot convert value "is the number I like to use" to type "System.Int32". Error: "Input string was not in a correct format."
At line:1 char:1
+ 1 + ' is the number I like to use'
+ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    + CategoryInfo : InvalidArgument: (:) [], RuntimeException
    + FullyQualifiedErrorId : InvalidCastFromStringToInteger 

The addition operator may be used to add single elements to an existing array. The following expression results in an array containing 1, 2, and 3:

@(1, 2) + 3 

Joining arrays with the addition operator is simple. Each of the following three examples creates an array and each array contains the values 1, 2, 3, and 4:

@(1, 2) + @(3, 4) 
(1, 2) + (3, 4) 
1, 2 + 3, 4 

Hashtables may be joined in a similar manner:

@{key1 = 1} + @{key2 = 2} 

The addition operation will fail if keys are duplicated as part of the addition operation:

PS> @{key1 = 1} + @{key1 = 2}
Item has already been added. Key in dictionary: 'key1' Key being added: 'key1'
At line:1 char:1
+ @{key1 = 1} + @{key1 = 2}
+ ~~~~~~~~~~~~~~~~~~~~~~~~~
    + CategoryInfo : OperationStopped: (:) [], ArgumentException
    + FullyQualifiedErrorId : System.ArgumentException

The subtraction operator may only be used for numeric expressions. The results of the following expressions are 3 and -18, respectively:

5 - 2 
2 - 20