The plot involves Rimuru's journey from being a rather weak (initially) slime to becoming a very powerful being. He meets many interesting characters. For example, there are the goblins he befriends early on. Rimuru helps them improve their living conditions. As the story progresses, he also has to deal with the threat of other more powerful monsters and even some human kingdoms that view his growing power with suspicion. In general, it's a story full of adventure, growth, and the forging of relationships in a magical and dangerous world.
A sloth's back story often involves its evolution to adapt to a slow - paced, arboreal lifestyle. Sloths evolved to move slowly in the trees to conserve energy. Their unique body structure, like long limbs and curved claws, developed over time for hanging and moving among branches. They also have a slow metabolism which is part of their story of survival in the rainforest environment.
The main character is Rimuru Tempest. He is a slime who has unique powers and a great deal of charisma. Another important character is Veldora, the storm dragon who befriends Rimuru. There is also Shizu, a human who has an impact on Rimuru's early development. And then there are characters like Benimaru, a powerful ogre who becomes one of Rimuru's loyal followers.
The Tensura web novel follows the story of Rimuru Tempest. He starts as a normal salaryman in Japan but is reincarnated in a fantasy world as a slime. Rimuru then begins to build his own nation and gathers various powerful and unique beings around him. He also gets involved in many power struggles and battles against other strong entities in this new world.