Without getting too long winded or technical:
They give you the option of dialling in the compression you want (using the peak reduction dial, 0 being none and 100 being maximum compression) either individually (L)left and (R)right channel separately or (=)equally. You can also adjust the gain (for volume only, doesn't affect compression) on the (L)left or (R)right channel separately or (=)equally in the same way depending what setting is selected.
The link button works in (L)left/(R)right mode only and if enabled equalises the amount of compression to the left and right channels when the (=)equal button is not selected, it doesn't affect the gain which can still be set individually left or right with the link button on.
OR
You can put the plugin in (M)mid/(S)side mode and compress the (M)mid channel or (S)side channel separately or (=)equally in the same way and also adjust the gain on the (M)mid channel or (S)side channel separately or (=)equally.
Load a full song file into your DAW and add the plugin, turn off link and select L and adjust the peak reduction to 60%, then select R and you'll see the peak reduction dial is now set differently for the right channel (the right channel will also sound louder) Hit link and you'll hear the compression equalise.
Reset the plugin and turn off link, select L and turn the gain up to 50, and you'll hear the left channel getting louder. Select R and you'll see the gain dial set differently. Hit link and nothing happens, the L channel is still louder as the link button doesn't affect the gain.
Not the clearest explanation perhaps but I hope it helps.
Then have a play with the (M)mid (S)side settings in a similar way and you'll hear what they do.
