A context window is how much information an AI can consider at one time. It can include your request, earlier messages, instructions, and information supplied by the app.
Imagine putting notes on a desk. The desk can hold only so many, so an older note may no longer be in view when the desk is full. In a long conversation, an AI may not be able to consider every earlier detail at once.
If one detail is important, repeat it or point the AI to the right file. The limit is not the only reason an AI may miss something, but clear reminders can help.