Steganography is technique of hiding one file (e.g secret.txt) to another file (eg catpicture.jpg). Example in Linux terminal which directory is in folder where is your catpicture.jpg:
1. echo "Very secret text" > secret.txt
2. 7z a secret.7z secret.txt
3. cat secret.7z > catpicture.jpg
And thats it, to be sure that there is secret.txt in your catpicture.jpg just move catpicture to other folder cd into that folder and type:
4. 7z x catpicture.jpg
You should see your secret.txt file.