Descartes talks about clear and distinct perceptions, in which clear means 'what is present and accessible to the attentive mind' and distinct means 'being clear and sharply separated from all other perceptions so that it contains within itself only what is clear'.
The clear part I can understand, but I have difficulties with really getting the distinct part. In the Principles of Philosophy, Descartes does mention an example about pain that shows that a perception can be clear without being distinct, but this doesn't really help me either.