From Matthew Plese at A Catholic Life:

What Are Near Occasions of Sin?

A near occasion of sin is specific to each individual, encompassing situations and circumstances that present a temptation to commit a mortal sin. Some are fairly common, such as being alone with an attractive member of the opposite sex. Others are less common but still relevant, like going to a bar if one has an alcohol addiction. It is primarily the individual’s responsibility to avoid deliberately placing himself in near occasions of sin that pertain to him.

Fr. Dominic Prummer, in his Handbook of Moral Theology, describes occasions of sin as following:

“Penitents are said to be in the occasion of sin when they are in a proximate occasion of sin. An occasion of sin is any extrinsic circumstance (person, thing, book, play, etc.) which offers a strong incitement to sin and a suitable opportunity.”

Fr. Prumer also separates occasions of sin into several categories: remote and proximate. The latter is further divided into absolutely proximate or relatively proximate. Occasions of sin are also classified as free or necessary, depending on how easy it is for one to avoid them.[4] For example, it is easy for a married man to avoid going out on Friday night with a woman who is not his wife. On the other hand, a young man and woman who are courting chastely and discerning marriage will nonetheless face a certain amount of necessary proximate occasion for sin. They are obligated to do their best to limit the strength of these temptations, which is one reason why Catholic tradition has always favored the practice of chaperones.

Avoiding Proximate Near Occasions of Sin Is Absolutely Necessary for Valid Confessions

Why does this matter? Because it affects the validity of the Confession. Fr. Prummer mentions the following rules for those in occasions of sin.