What does the statement immune system has a memory mean?

The statement "Immune system has a memory" means that the immune system has the ability to "remember" and retain information about previously encountered pathogens (disease-causing microorganisms like bacteria, viruses, fungi, or parasites) or foreign substances. When the immune system encounters a pathogen for the first time, it mounts an immune response to eliminate the pathogen. Some specialized immune cells, such as memory B cells and memory T cells, store information about the encountered pathogen. If the same pathogen is encountered again in the future, the memory cells can quickly recognize it and mount a much faster and more effective immune response. This immunological memory ensures enhanced protection against re-infections with the same pathogen.