What does "unlocked" mean? (Tech New Guy Question)?

[Android Central Question]

I need a new phone and see "unlocked" phones online. Not sure what it really means. I use Walmart Family Mobile which uses T-Mobile. I have a Samsung On5 and its 8GB. I can't add apps because it has no storage left after adding just a few for Smart-Home items.
I just want to get a phone and put in the SIM card without issues.

Any input is appreciated.

 

[Kodak2]

Unlocked means that the phone is not locked to only I carrier, an unlocked phone will accept most carriers sim cards automatically.