What does `echo ${!a}` do and why?

The following bash script prints out c, but I'm not sure why.

a=b
b=c
echo ${!a}

I believe that !a will be converted to the last command which began with a, so in this case that would be a=b. This should mean that the last command is equivalent to the following.

echo ${a=b}

However, that prints b, not c. Is anyone able to explain why this is the case?