It's mainly about the daily adventures and interactions of Rimuru and other characters in the slime world. It shows their lives beyond the intense battles, like how they build their community, deal with various funny incidents, and develop relationships with one another.
The main character is Rimuru, the slime. Alongside him, there are characters like Shuna, who is sweet and has her own skills in things like cooking. Gobta is also there, a rather comical character who often gets into funny situations. These characters interact with Rimuru and each other, and their relationships are what make up a large part of the charm of the 'slime diaries light novel'.
Well, one obvious difference is the medium. Manga is visual, so you can directly see the slime's appearance, the expressions of characters, and the action scenes. For example, in a slime battle scene, the manga can show the slime's movements vividly. However, a light novel uses words to describe all these. It might describe the slime as a gooey, translucent blob that pulsates with a strange energy.
Slimes in these novels are often depicted as having a gelatinous, amorphous form. They can usually change shape to some extent, which gives them an advantage in various situations, like squeezing through small spaces.
Well, without more context, it's difficult to say exactly. 'Slime' in light novels often refers to a type of creature, which can have various forms and abilities. The '300' might be a volume number, a chapter number, or something else entirely. It could be a story where the slime is the protagonist and there are 300 important elements in the plot, like 300 quests it has to complete or 300 different types of magic it encounters.