-f Format operator

Format a string expression.

Syntax:
       "String with placeholders" -f "Array of values to place into the placeholders"

       'Filename: {0} Created: {1}' -f $_.fullname,$_.creationtime

       "{I,A:FS} {I,A:FS} {I,A:FS}.." -f "string0", "string1", "string2"...

Place {0} {1} etc. into the string as placemarkers where you want the variables to appear, immediately follow the string with the -f operator and then lastly, a list of comma separated variables/objects which will be used to populate the placemarkers.

Key:
   I   Index of the item to display, 0,1,2 etc.

   A   Alignment. 
       A positive number will right align n characters. 
       A negative number will left align n characters. 
        so {2,-25} will allocate 25 characters of horizontal space on the line, even if the string is only 1 character long. 

   FS  An optional Format String that acts on the item depending on its type (not case sensitive).

       Valid format strings:

:c	Currency format (for the current culture)
:d	Padded. (:dP precision=number of digits); if needed, leading zeros are added to the beginning of the (whole) number.
:e	Scientific (exp) notation
:f	Fixed point :f5 = fix to 5 places
:g	Most compact format, fixed or sci :g5 = 5 significant digits
:n	Number (:nP precision=number of decimal places), includes culture separator for thousands 1,000.00
:p	Percentage
:r	Reversible Precision
:x	Hex format
:hh :mm :ss	Convert a DateTime to a 2 digit Hour/minute/second "{0:hh}:{0:mm}"
:HH	Hour in 24 Hour format
:dd	Day of Month
:ddd	Convert a DateTime to Day of the Week
:dddd	Full name of Day of Week
:yyyy	Full year
#	Digit Place Holder

Some of these letters (:d, :D, :f, :F, :g, :G and :r, :R) also perform double duty as date patterns which can lead to some confusion.
When applied to a date they will act as date patterns and they are then case sensitive: Full list of Date Formats

The -f format string can be surrounded by either single or double quotes.

Static text or more complex expressions may be included before or in-between the -f {format strings}
The -F operator has equal precedence with Arithmetic operators, * / % + - etc, see About_Operator_Precedence
When operators have equal precedence, PowerShell evaluates them from left to right.

Examples:

Display a number to 3 decimal places:
PS C:\> "{0:n3}" -f 123.45678
123.457

Right align the first number only:
PS C:\>"{0,10}" -f 4,5,6
         4

Left and right align text:
PS C:\> "|{0,-10}| |{1,10}|" -f "hello", "world"
|hello     ||     world|

Display an integer with 3 digits:
PS C:\> "{0:n3}" -f [int32]12
012

Separate a number with dashes (# digit place holder):
PS C:\> "{0:###-##-##}" -f 1234567
123-45-67

Create a list of 100 names with a padded suffix no. (Name001 → Name100):
1..100 | % { 'Name{0:d3}' -f $_ }

Convert a number to Hex:
PS C:\> "{1,10} {0,10} {2,10:x}" -f "First", "Second", 255
    Second     First        FF

Display filenames and creation time:
PS C:\> Get-ChildItem c:\docs | ForEach-Object {'Filename: {0} Created: {1}' -f $_.fullname,$_.creationtime}

Display only the Year from a date time value:
PS C:\> "{0:yyyy}" -f (Get-Date)
2018

Display the hours and minutes from a date time value:
PS C:\> "{0:hh}:{0:mm}" -f (Get-Date)
17:52

Reverse the order of display:
PS C:\> "{2} {1,-10} {0:n3}" -f [math]::pi, "world", "hello"
hello world 3.142

Display a number as a percentage:
PS C:\> "{0:p0}" -f 0.5
50%

Display a whole number padded to 5 digits:
PS C:\> "{0:d5}" -f 123
00123

“I skate to where the puck is going to be, not where it has been” ~ Wayne Gretsky

Related PowerShell Cmdlets:

PowerShell Operators - Format strings and arrays.
Variables - PowerShell Variables and basic Mathematical operators (+ - = /).
Pipelines - Pass objects down the pipeline.