A small, cheap notebook for writing quick notes in — the kind you keep by the phone or carry in a bag, not the kind you write an essay in. The word is British and, more than that, it is strongly Scottish: in Scottish schools a jotter is the standard word for the exercise book a pupil writes their schoolwork in, and Scottish children say it every day. In England it is understood but sounds slightly old-fashioned, and in the United States and Canada people generally do not use it at all — an American would say notepad, notebook, or memo pad. So it is a word worth knowing when you read or listen, especially anything Scottish, but not one you need to use yourself. The word 'jotter' can also mean a person who jots things down, but you will almost never meet that use.