Ratio decidendi is a Latin phrase referring to the "rationale for the decision."
In the common law system, the law is not so much a set of rules as a series of decisions made by courts. When a lawyer seeks to understand what the law is, they will study court opinions written by judges to find out what the court's ratio decidendi was in each case.
The rationale for a court's decision (ratio decidendi) is an essential part of predicting what the law is in any specific situation. When lawyers advise clients or argue a case in court, they must study the history of court decisions in matters similar to the case at bar to understand what the precedent is.
In a common law system, judges compare the situation in the case at hand to similar cases that have already been decided to make their rulings. Often, specific language in an opinion written by a judge, which forms the ratio decidendi, will become the law for those situations.