getting "almost out of space"

i'm getting a message with" You're almost out of space! Upgrade Now." 
what space i'm taking?
and how this will effect my forms submission ?
thanks for you help 

Hello,

Free users are limited to 100 submissions per month, your form have received 106 submissions so far. You may consider to uprade the account to a paid subscription or to wait until the first day of the  next month when submissions counter will be reset.

If you have additional questions, please let us know.

Kind Regards.