When mixing vocals we should always have an end goal in mind.
What are we looking to do with the vocal? Is there anything the vocal lacks that we can improve? Is there anything we love about the vocal that we would like to accentuate?
Compression is automated volume adjustment. You set the parameters with the attack and decay (when compression will start and stop), how much it will be adjusted via the ratio, and lastly when the incoming signal will trigger the volume adjustment aka the threshold.