In psychological research a baseline is a measurement of the variable of interest at the beginning of treatment or a study that is used to compare to later measurements in order to judge the effectiveness of the treatment or conditions.
For example, a clinical psychologist wants to see how effective a new type of therapy is for anxiety. For their study, they measure the anxiety levels of the participants by using a specific test that has been shown to be effective at measuring anxiety. The clinician then starts the new therapy method and uses the same anxiety measure every month and compares the scores to the baseline measurement in order to gauge its effectiveness as a treatment. If the participants’ anxiety levels have been reduced from the baseline score then the therapy could be said to be effective. If the anxiety levels are unchanged or higher then the clinician may want to not use this method in the future.