Shift left
From WikiManual
It picks two stack values and shifts the first one's bits the amount specified by the second one.
Shifting 1 by x will return the x power of 2.
Shifting x by 1 is the same as multiplying it by 2.
Shifting x by 2 is the same as multiplying it by 4.
Shifting x by 3 is the same as multiplying it by 8, and so on.
