"A baseline grid is a grid of horizontal lines that are evenly spaced across your document. Without the grid visible it can be tricky to make your text perfectly aline across your document.
A baseline grid is particularly handy for multi-column or spread-based layouts, like magazines, flyers or EPUBs, where you have multiple text frames lined up next to each other. Overall, it promotes a more uniform, professional look across your InDesign documents, so it’s well worth doing."