Are probation periods to start a job standard procedure?

I recently signed an offer letter for a job and I saw the probation period language (it was technically like orientation and review period). My period is 90 days and I’m kinda worried about it. Is this standard? This is my first job with a proper HR department and hiring procedures so I assume it’s normal. Also, we have at will employment so employers can fire you at any time regardless.

usa jobs
usa jobs resume
usa hotel jobs
usajobs
usa jobs federal government
usa job in ksa
usa jobs
usa jobs login
usa jobs gov
usajobs.gov
www.usajobs.gov
usajobs.com
usajobs
usajobs.gov official