Your discretionary income is that extra money that you don't have plans for after you must have taken care you your basic needs and monthly expenses. Some people use their extra money to gamble, club, drink, womanize and many more but it's wise for you not to use this funds for consumption purpose. Rather, invest it into bitcoin and save the value of your funds for the future.
Like what the name suggests, "discretionary" means personal judgement. By applying it to your income, if you have discretionary income, this means that you are free to use that portion to whatever nature you desire.
Like what most have mentioned, you may definitely use it for your savings, entertainment, food, investment, etc. Preferably, however, almost everyone recommends using it primarily for cryptocurrency investments for long-term HODLing.
Now the reason on why discretionary income is to be preferred because this is part of your income which arises your savings. After allocating all the expenses, you are left with a surplus of money that you can further increase for future use.