log + & log - just means the response to input wont be equal on the output. log is short for logarithmic and essentially denotes an exponential response to some input. as the diagram shows above, a logarithmic response curves in a predictable manner. you could say that a log - response curves towards the max output value late during control input, and log + starts out responding very quickly during control input and then levels off before maximum output value is reached. in both cases, the curve is symmetrical and can vary in degree of severity. a slight log curve will only deviate from a linear response curve by a little and will look like a bow and arrow like the above diagram does.
all this basically means that if your faders are set to a log - response curve, when you start moving the fader from the minimum towards the maximum, it will not change the output value much until you reach about half way and then the output value will rise very quickly towards maximum. so, before half way, the response is less than linear and after half way the response is more than linear.
where this applies to synth parameters is when you have a control like an envelope or lfo that is routed to say, a filter, they act as your input control (the fader was our input control in the example above) and if you set a logarithmic response curve, in the case of log - your filter will respond more 'dramatically' where as with log + it will respond less dramatically. of course 'drama' is relative to the filter type and intended application. oh and your artistic vision.
in case you were wondering, a linear response is: what you input as your control mirrors the output. the output can be scaled, but basically every point along the input equals another point in the output that is scaled an exact amount along the entire input range.
hope that makes sense.
