what do different characters in tload output mean?

Does anyone know what the different characters in the tload graph output mean? Mostly there are * but I also = and -.

It's just the graphical (weird) representation of the last minute load average of the system over the time.

The "*"s are just the representation of the level of load average. Understand it as a clasical "area graph". The "-" sings just define the unities of load, and the "=" substitute the "-" when the bar passes the line that marks the unit of the graph. You can change the scale with the "-s flag".

A visual example may be clearer:

------------------------------------- load 3

                     *  
                     **
--------------------====------------- load 2
    **             ******   **
   ******         *************
  *********       **************
--=========-------==============----- load 1
*************************************
*************************************
*************************************
************************************* load 0

In case don't know how to interpret the load average numbers check this wikipedia link