Steganography is the practice of concealing a message within another message or a physical object.
For example,
you have a text message that contains "HELLO" to be sent.
Instead of that message, you push each letter one step forward, i.e., a to b, b to c, and so on.
So the new message is, "IFMMP".
Now you send that message to your receiver. He knows the algorithm. And third-party intruders doesn't.
Here, only your friend knowing technique can read the message, shifting each letter one step backward. Which again reads, "HELLO". And it's just "IFMMP" for third-party intruders.
Now, this is a very basic example of Steganography. Here we're talking about hiding messages or even files in another file, which only can be revealed using the reverse algorithm of the algorithm which is used to hide data.
Steganography is nothing new tech or term. It has started early before in history (440 BC). Histiaeus (an ancient Greek ruler) sent a message to his vassal, Aristagoras, by shaving the head of his most trusted servant, "marking" the message onto his scalp, then sending him on his way once his hair had regrown, with the instruction, "When you reach to Miletus, ask Aristagoras to shave your head, and look thereon." That's a Steganography.
If you remember, you must have used invisible ink to write something as a kid. And later, when the paper was heated, the message was revealed. That's a Steganography.
Also, you must have heard, some Pincode is separated into three-part and given to three different people. When they all enter the part of their pin code, the message (secret) is revealed. That's a Steganography.
In summary, it's all about hiding messages in another file using some sort of encryption algorithm. That message could be text, audio, files, or image. When a decryption algorithm is used with an encrypted file, the actual message or file is revealed.