Habituation
Habituation is when an animal stops reacting to something because it has seen or heard it many times.
Think of it this way
A smoke alarm hangs on the kitchen wall. The first time it beeps during a test, everyone stops and looks up. After it beeps the same way each morning for a week, no one looks up. Animals do the same thing: they stop reacting to a harmless signal they have met many times.
Explaining habituation by grade level
A betta fish sees itself in a mirror and flares its fins wide. It thinks the reflection is another fish. After many days, the betta stops flaring. It learned that the mirror is not a real threat.