Vari-mu compressors are tube compressors which usually work with a fixed ratio at which, depending on the amount of input and output signal, we determine the amount of compression and the volume of a track in the mix.
Within the different models, we can see that they have a medium speed, being able to have quite fast speeds in certain models. The use of compressor aesthetics usually helps to create a dynamic coherence, since they usually have a fast attack and a slower release to be able to pack or better control the mixes, the masterings or even an individual section or track in the song.
There are many types of different colors in this type of compressors since there are many brands that usually make them, starting with Universal Audio, Manley or the Fairchild line.
If this is your first time using this type of compressor, many brands like Waves have very versatile emulations, which can be tested on string sections in styles like pop. In addition, each model has its characteristics, so it would be necessary to study the manual of each one a little to discover and get to know the equipment better.