Just a variable that holds multiple values.
DIM A[4] //create var 'A' with a size of 4. Array indexing starts at 0 so the total size is N-1.
A[0]=1:A[1]=3:A[2]=7:A[3]=9
// Now if you did
X=A[1]*A[2]
X now equals 21.
A variable is like a sheet of paper that's been assigned a name (like MONEY, HEALTH, AMMO). You can write & read a different data (number or letter) on each sheet and reference the data by using the name assigned to the sheet.
An array is like a booklet of those sheets that are referenced by booklet name and page number. TREASURE[5] would be for page 5 of the booklet named TREASURE.