What is an unlocked phone?

[chicrose]

I plan on buying a "unlocked cell phone" from walmart but im not sure what it is. I have boost mobile plan so if i put my sim card in the unlocked phone can i now use it with my boost mobile sim card? will I still have the same features as my old boost mobile phone like not being able to watch youtube videos on the unlocked phone like i cant on the boost mobile phone i have now. I know it cant be this easy to just change features on a phone so can you please help me? Thanks