In a database what is a field width?

It is the amount of space that you want to allow for some data. Different kinds of data will take more space than others, so you should define how much space is needed. So if you had a field for Gender, then the values would be Male or Female. So a maximum of 6 letters is needed, so you could specify that as the field width. If you set it larger than that you would be allocating space that was not needed, and you would be wasting space. For other things, like names or address lines, you would need more space than for your gender field, so you would set them to have more space. Some fields have their widths automatically set, like date fields, so you don't need to specify anything.