First, there are a couple of types of comments in PowerShell. There are line comments and block comments. Line comments are denoted with a #. For example, you could include the following comment.

#
#    Hello. This is a comment
#

Everything after the # will not be executed by PowerShell.
The second kind of comment is a block comment. A block comment allows you to comment sections of code with start and end tokens. To start a block token, you should use the <# token. To end the comment, use the #> token. For example, we could create the following comment.

<# 
     Hello. This is a comment.
#>

Everything between the <# and #> will not be executed by PowerShell. You can also use block comments directly in-line with other scripts. For example, you could include a block comment right in a cmdlet call.

Get-Process <# -Id 12 #> -Name 'Notepad'