Dave’s right. PowerShell ISE and other editors highlight matching brackets to give you a visual cue so that you can match brackets more easily. It’s a very helpful feature when using multiple sets of brackets nested within one another.
The error you receive indicates that PowerShell was unable to find a computers.txt file in your current directory when you ran that script. If you modify the script to use an absolute path to computers.txt (i.e. C:\MyFolder\computers.txt instead of .\computers.txt), or if you change your current working directory to the directory where computers.txt is stored, then it should run just fine.