A format is a way of encoding the information of an asset so that it can be stored and read later. For example, a texture may be stored as a Bitmap (BMP), Portable Network Graphic (PNG), or Targa (TGA). Game levels can be stored using RON, JSON, TOML or any other suitable encoding.
Each format has its own strengths and weaknesses. For example,
RON has direct mapping from the storage format to the in-memory object type.
JSON is widely used, so it is easy to find a JSON parser in any programming language.
TOML is easier for people to read.