What is the meaning of the "dup" command in assambler?

I am studying assembler and I have an exercise that is written like this:

    GODEL EQU 10    
    DSEG SEGMENT
    BLOCKA  DB 10H,20H,30H,40H,50H,60H,70H,80H,90H,95H 
    BLOCKB  DB 20H,30H,40H,50H,60H,70H,80H,90H,95H,95H 
    BLOCKC  DB GODEL DUP(?) 
    DSEG  ENDS 
    SSEG   SEGMENT STACK                
           DB 100H DUP(?)   
    SSEG   ENDS 
    CSEG   SEGMENT            
           ASSUME CS:CSEG,DS:DSEG,SS:SSEG 
    BEGIN:           MOV AX,DSEG                        
                     MOV DS,AX                        
                     MOV CX,GODEL                       
                     MOV BX,0000H   
    AGAIN:           MOV AL,BLOCKA[BX]                        
                     ADD AL,BLOCKB[BX]                        
                     MOV BLOCKC[BX],AL                       
                     INC BX                       
                     LOOP AGAIN                       
                     MOV AH,4CH                        
                     INT 21H              
   CSEG              ENDS                
   END               BEGIN

my question is that I still do not understand what the DUP command does