Please help me understand the steem currency. Mainly the bit in brackets under my steem power.

Hi all

I am new here.

I think I am getting my head around the difference between steem, steem power and steem dollars.

What I don't understand is the number in brackets below my steem power.

I.e.

Steem power =

0.5 STEEM
(+28.240789 STEEM)

What is the bit in brackets the 28.24 where did it come from and what happens to it.

Kind regards

And thanks in advance