A vocal rider, in the context of audio engineering and music production, refers to a dynamic equalization or compression setting used to control and adjust the level of vocals in a mix. It's essentially about managing the vocal levels to ensure they sit well in the mix, often requiring precise adjustments to avoid overpowering other elements or getting lost.
If you do not have the budget for the official Waves plugin, several developers offer free tools that automatically ride and level vocal gains without adding heavy compression: waves vocal rider download hot