Basically sages are people who manipulate natural energy.
Naruto and Jiraiya are both Toad sages meaning they learned of natural energy manipulation from Toads. Naruto is a perfect sage, but Jiraiya undergoes a partial frog transformation in his feet and nose (basically meaning he isn't a perfect sage).
Kabuto went to the snake Sage where he evolves the way of the snake into the dragon for Dragon Sage mode.
Juugo is like a pseudo Sage as his clan can use natural energy, but it overly reforms their body and costs them self control.
Basically natural energy bolstered physical strength allowing a small toad to lift up Gammabunta (big toad lol) and throw him a fair distance.
Also one's Jutsu are amplified giving them greater strength.
For example, without sage mode, Fuuton: rasenshuriken is just a wind element Rasengan meaning he has to hit the enemy with it as a punch; however, in sage mode, Naruto can actually throw it!
But yeah Naruto, Jiraiya, and Kabuto....technically Juugo.
