Fluid layout is an alignment tool that lets you align elements so that they automatically adjust their proportions and layout as screen sizes change. Fluid layout alignment works with both percent-based and fixed-size elements, as well as combinations of both. To use fluid layout, you need to be working with an ad type that supports multiple screen sizes. Fixed-size ads won't benefit from fluid layout.
Use fluid layout when aligning elements
Checking Fluid layout before using any of the alignment tools creates a percentage-based alignment that will keep the elements relatively aligned to each other and to the enclosing container as the container changes size.