An Inside View on a topic involves making predictions based on your understanding of the details of the process. An Outside View involves ignoring these details and using an estimate based on a class of roughly similar previous cases (alternatively, this is called reference class forecasting).
For example, someone working on a project may estimate that they can reasonably get 20% of it done per day, so they will get it done in five days (inside view). Or they might consider that all of their previous projects were completed just before the deadline, so since the deadline for this project is in 30 days, that's when it will get done (outside view). Another term for taking an outside view is reference class forecasting.
The planning example is discussed in The Planning Fallacy. Possible limitations and problems with using the outside view are discussed in The Outside View's Domain and "Outside View" as Conversation-Halter. Model Combination and Adjustment discusses the implications of there usually existing multiple different outside views....