Steganography comes from the Greek word Steganographia (Στεγανογραφία). What it means is simple: Concealed writing. Someone could also say “Writing in secret”. With steganography the fact that some information is hidden, is not supposed to be obvious. In contrast with cryptography, where information is encrypted and one can see it but is unable to read it, with steganography no one should really know that there is any information to be found at all!
The word has a general meaning and steganography can be implemented in many ways, however, is mostly used in Cybersecurity for describing the act of hiding some digital information inside digital images.
A digital image is a piece of digital information. Digital information is a sequence of digital bits. Digital bits are nothing but 0s or 1s in a unique combination.
When the quality of the image is high, the amount of information stored within, is big.
Would that mean that if some of its bits are altered, we would not notice? That is true. Most probably we would not. Would the image though be the same? No, it would not. However, images, especially photographs, with a lot of colours, can also have a lot of noise. Additionally, if there is only one copy of the image available to someone and this is the altered one, then it would be impossible for them to tell as they would not have a reference to ever make a comparison.