Ok on my sidekick 2008 what does the plus sign mean in the sent messages?

My phone broke and i have to use my Sidekick 2008 now and when i send a text messages there is a plus sign in the "sent folder" next to the number i sent it to. What does this mean and on some texts they dont have one. Does this mean if they have a plus sign they got the message ? and i know it has nothing to the with the ones before a number