A delimiter marks the boundary between records or fields.
For example
In a text file each line can be delimited by the carriage return, line feed.
line1 CRLF
line2 CRLF
You cannot see the CRLF characters in a text editor, but if you open the file in binary mode you will see 0x0D and 0x0A characters at the end of each line.
In a csv file, each field is delimited by a comma.
field1, field2, field3.
The delimiter tells the parser when to stop reading one record or field and when to start the next one.