Would someone explain what "up sampling" means and how it can affect the sound?

Hello.
I am looking at buying a DAC and after reading a few articles, there seems to be a question that comes to mind: what is up sampling, and does it affect the sound ? Some audio companies have up sampling in their DAC and some do not.
I just want to make an educated decision about what DAC I purchase that, hopefully, will not be obsolete in a few years.
Thank you for all your comments and answers.