The serial port receives a series of bytes that are interpreted by your code. The bytes have no inherent meaning - you give it the meaning. The byte you are receiving in your example are being interpreted as chars, in other words ASCII codes. The byte gives you a number between 0 and 255. If you send two bytes one could be the low and the other the high byte of a 16 bit value or in fact they may be 2 ASCII characters.
If you enter data into the serial monitor then this will be sent as ASCII chars. However, between devices there is no restriction on what the bits transmitted mean. All you need to do is agree at both end on the protocol that will be used to send, receive and interpret the data.