In Sutta pitaka we find two Suttas namely Jhana sutta and Samadhi sutta. What is the difference?
Samadhi is a general term for concentration. Jhanas are stages you achieve in Samatha meditation when your concentration reaches a certain level.
To give an analogy, Samadhi is like the temperature of ice. When the temperature reaches a certain level, it becomes water. That's like reaching the first Jhana. If you keep heating, it will become steam. That is like reaching the second Jhana and so on.
In tipitaka, buddha and sāvaka often use them as a synonym for each other.
Because when mind has just one focusing (jhayatiti jhāna) is when mind has concentration (samadahatiti samādhi).