Storage value means retrieving a value from player variable storage.
Every player character has a total of 65535 (or 65534, not sure) available variables that can be used for storage of numbers, these variables is defined by their ID, which is 1 - 65535 (or 65534).
That way you can store data on the player which allows different scripts to pass information to one another. For example, make one script store a 1 inside the variable #500, and then have another script read that variable, and if the player has a 1 in there, allow him to proceed the quest.
To read/write/edit storage values, you use the following commands:
getPlayerStorageValue(uid, valueid)
setPlayerStorageValue(uid, valueid, newvalue)
The first one one would read the variable with the ID# specified using valueid.
The second one would set the variable with the ID# specified using valueid, to the number specified in newvalue.
For example:
getPlayerStorageValue(cid, 500)
This would check what number is in the variable ID# 500 on the character executing the script.
setPlayerStorageValue(cid, 500, 1)
This would set the number in variable ID# 500, to "1" on the character executing the script.