The field name is the name of the field. Field type is the storage form of field content. The field length is the length that the field can hold. A long integer is a long numeric form without a decimal point. Double precision is a kind of floating-point type, which is simply a long number with decimal point. The length of the field depends on your needs, such as how many decimal places are needed for double-precision types; How many words do you want to write in the string type?
This is the basis of computer, and I can only say it briefly. Just buy an introductory database tutorial and have a look.