Mash thickness, also called the water-to-grain ratio, describes how much strike water is used per unit weight of crushed grain — typically somewhere in the 2.5-3.5 L/kg (1.2-1.7 qt/lb) range for most homebrew recipes, though real recipes vary meaningfully outside that window for specific reasons.
A thinner mash affects more than just how it feels to stir
A thinner mash (more water per unit of grain) generally lauters faster and, some brewers report, slightly favors beta-amylase activity over alpha-amylase — pushing fermentability marginally higher, though mash temperature remains the dominant lever for that outcome by far. A thicker mash concentrates enzyme activity and is the traditional choice for a decoction mash's initial rest, since a thicker mash holds heat more efficiently when a portion of it gets pulled and boiled separately.
Thickness changes your actual strike temperature, not just your rest
Because water absorbs and releases heat differently than grain does, mash thickness is one of the direct inputs the strike water calculator needs alongside grain weight and target temperature — the same target mash temperature needs a different strike water temperature depending on how thick or thin the mash ratio is, which is a common source of a mash coming in off-target for a brewer who reused an old strike temperature without re-checking it against a new ratio.