A rater is a co-worker or colleague with whom you work closely on a regular basis. A rater will provide you with feedback by completing a survey using what they already know about working with you. They will answer the same set of questions that you answer about your skills, strengths, development needs, etc.
It is preferred that you assign your raters as early in the process as possible to allow them adequate time to give you accurate feedback. Encourage your raters to complete the surveys as soon as possible before the deadline to ensure your data is processed and ready for your program. The time and consideration that raters put into survey responses will directly impact the quality of feedback you receive.