Numbers 5:15
Then shall the man bring his wife unto the priest, and he shall bring her offering for her, the tenth part of an ephah of barley meal; he shall pour no oil upon it, nor put frankincense thereon; for it is an offering of jealousy, an offering of memorial, bringing iniquity to remembrance.
The law applied when a husband, moved by jealousy, brought his wife before the priest because he suspected her of unfaithfulness yet had no direct evidence against her. The husband was required to bring an offering for her consisting of a tenth of an ephah of barley meal, upon which he was to pour no oil and put no frankincense. This offering was an offering of jealousy and a memorial offering bringing iniquity to remembrance.
Its purpose was not to make atonement for sin but to bring hidden iniquity to remembrance and to disclose it. In this it illustrates the broader way God appointed memorials as means of bringing matters to remembrance. The bitter water connected with the rite had no harmful effect upon an innocent woman, but upon a guilty woman it brought a dreadful and speedy death. The chapter to which the ordinance belongs thus exposes God's abhorrence of sin and warns against its destructive course and fearful end.