Simple Definition:

An encoder is a device or program that converts information or data into a specific format or code [a system of symbols used to represent information]. It is used to transmit or store the information in a more efficient and secure manner. For example, a video encoder converts video data into a compressed format to reduce its file size and enable faster transmission over the internet.

Very Simple Definition:

An encoder is a tool that changes information into a different form, making it smaller or easier to send. For example, a video encoder makes videos smaller so they can be sent quickly over the internet.