Each record inside any collection has some exact structure. So, you can define a list of properties with corresponding data types. A record type is such kind of structure. Each record has a particular record type. Moreover, each collection can collect not only records with the same type but with multiple ones too.
By default, a new collection creates with single record type support if you need support for different types you need to set the corresponding option.
Also, each collection must contain at least one record type. When you are creating a new collection you must also fill "Single record name" field which is a name of that record type. Depends the value of option "Multiple record types" you can see a collection interface in different design and also has different "Options" menu.
You can use corresponding interface for creation new record types.
There are two categories of record types are supported:
Ordinary record type
Ordinary record types could be used if your records are completely different. But in many cases, if you need multiple record types, you actually need different data schemes but with some common properties. For example, if you have a collection with some products you need an information about prices or stock information, some content fields (description, etc.). Or, if you have a collection of some project logistic equipment, like cranes, trucks, trailers, barges etc., you need also common properties like description, location, status, etc. So, in such cases you need create abstract record type with a set of commonm properties. Then you can create a list of child record types which are extensions of an abstract type.