Magic Number is the unique number assigned to all open algo trades so that the algo “knows” which trades belong to its algo.
If you have TWO or more Expert Advisors (EAs) and they trade on the same symbol (e. g. EURUSD), they won't logically be able to track the orders as each EA would consider all of the open trades as its own. So you need a unique identifier for each EA to track all orders related to each individual EA.
This magic number is user definable and will normally be a combination of user extern input + chart period # (in minutes) . The user must ensure that they make the input number is unique if running multiple EAs on the same pair.
The only time you will hear of this “Magic Number” is MetaTrader- 4 & 5. Although other platforms have similar problem with using multiple algo on the same symbol and a similar “Magic Number” unique identifier.