If

Conditionally perform a command.

Syntax
      if (condition) {commands_to_execute}
      [ elseif (condition2) {commands_to_execute} ]
      else {commands_to_execute}  

Key
   Condition  An expression that will evaluate to true or false,
              often utilising one or more comparison operators.

   commands_to_execute
              A powershell or external command to run if the condition is true.

Comparison operators:

-eq = Equals
-match = Match (Not quite as definitive as equals)
-ne = Not equal
-notmatch = Does not match
-gt = Greater than
-ge = Greater than or equal to
-lt = Less than
-le = Less than or equal to

Examples

Replace the text in the variable $MyDemoVar:

PS C:\>if ($MyDemoVar -like "*SS64*") {$MyDemoVar -replace "SS64", "Demonstration Example"}

Print the running services in green and stopped services in red:

PS C:\>get-service | foreach-object{ if ($_.status -eq "stopped") {write-host -f red $_.name $_.status}` else{ write-host -f green $_.name $_.status}}

You see things; and you say 'Why?' But I dream things that never were; and I say 'why not?' - George Bernard Shaw

Related:

ForEach - Loop through values in the pipeline
Comparison operators - Full list