Multiband compression is a tool that gives you more control over what gets compressed when you're, well, using a compressor (see our article What is Multiband Compression for more details). Usually the action of the compressor is universal in terms of the frequencies affected. You give the effect some parameters so that it knows where to kick in (that's the threshold of signal level that triggers the action), and then you tell it how much to reduce the signal level (turn it down) for each decibel (that's called the ratio - 1:1 means nothing gets turned down). Then the compression gets applied to any and every frequency that happens to cross the threshold.
But with multiband compression, you can apply different settings for different frequencies.
Here is an article that will show you some ways you can use multiband compression.