help me to understand linux shell script contains ${var[@]}

CWD=`pwd`
var=(SUUSERNAMECliqr SUPASSWORDCliqr ISSERVERENDPOINTCliqr APISERVERENDPOINTCliqr DOMAINCliqr EMAILCliqr ENDPOINTCliqr TETUSERNAMECliqr TETPASSWORDCliqr)
RSYNC=`which rsync`


# import environment
. $ENVFILE

# Copying JSON payload to local
$RSYNC -ar $CWD/deps/datadump/ $CWD/jsondata

# Setting up JSON DATA
for i in `ls $CWD/jsondata`
do
        for j in ${var[@]}
        do
                sed -i -e "s,$j,${!j},g" $CWD/jsondata/$i 2>/dev/null ;
        done
done

i have some jsonfiles like 1.json , 2.json ,3.json in the directory $CWD/jsondata/..

what i understands is the value SUUSERNAMECliqr substitutes as $SUUSERNAMECliqr  to the files 1.json , 2.json ,3.json ... this i understood from the command sed -i -e "s,$j,${!j},g" $CWD/jsondata/$i 2>/dev/null ; ......

but my confusion is how this for loop ... for j in ${var[@]}......substitutes values from the array declared  var=(SUUSERNAMECliqr SUPASSWORDCliqr ISSERVERENDPOINTCliqr APISERVERENDPOINTCliqr DOMAINCliqr EMAILCliqr ENDPOINTCliqr TETUSERNAMECliqr TETPASSWORDCliqr).......

i.e. how @ symbol  tells to check all the values in array i.e. for j in ${var[@]} ..... can any one explain me this please ?

for j in ${var[@]}
        do
                sed -i -e "s,$j,${!j},g" $CWD/jsondata/$i 2>/dev/null ;
        done