Quick meaning
Responding to an email so that every person who received the original message gets your reply.
Think of it like this
Like answering a question in a group meeting so everyone hears the answer.
Where you may see this
You may hear this in email apps, messaging apps, video calls, and workplace instructions.
Why it matters
Knowing the term makes it easier to message people correctly and avoid simple communication mistakes.
Say it simply
Reply All means: Responding to an email so that every person who received the original message gets your reply.