This is a fancy word for all the effects or complications, typically long-term, that occur as a result of a disease, trauma, injury or sometimes of a therapy/intervention/treatment. Sequelae (a plural word) differs from symptoms in that symptoms are features or characteristics of a disease while it’s occurring. Sequelae are what follow afterward, even after recovery, and generally refers to a separate (often chronic) condition or set of symptoms that are distinct from the original condition but were also caused by it.
For example, common symptoms of concussion are loss of vision, blurry vision, nausea, dizziness, etc. But possible long-term sequelae of concussion include loss of short-term memory, headaches, nerve damage, reduced focus/concentration, irritability, etc. Sometimes later sequelae may be used to retroactively diagnose an earlier disease or injury that had been missed.