The basis is "free association": you say whatever comes to mind without censoring or tidying it. The therapist listens carefully and sometimes offers an interpretation, a link between what you say and a meaning that may lie behind it.
"Transference" plays a central role. Feelings and expectations left over from earlier relationships carry over into the relationship with the therapist. The therapist attends to these, and to their own reactions in the session, and uses them to understand you better.
A stable frame, meaning a fixed time, fixed session length and clear rules, is itself part of the treatment. It creates a safe space in which difficult themes can emerge gradually and at a safe pace.