Mash out is a step some brewers add at the end of the main mash rest, raising the temperature to roughly 168°F (76°C) — hot enough to denature the enzymes still converting starch, halting further conversion, and hot enough to noticeably reduce the wort's viscosity, which makes lautering faster and generally more efficient.
Locking in the fermentability you already reached
Because beta-amylase and alpha-amylase are both still active at typical mash-rest temperatures, an extended lauter without a mash out can, in theory, let conversion keep drifting — usually a minor effect with modern well-modified malt, but a mash out removes the question entirely by shutting enzyme activity down at a known point rather than an unknown one determined by how long the lauter happened to take.
Skipping it is common, and usually fine
Plenty of homebrewers skip mash out entirely without a noticeable difference in the finished beer, particularly on a normal-length single-infusion mash with a reasonably quick lauter — it's a genuinely optional refinement rather than a required step, more useful on a longer or slower lauter, or when a brewer wants tighter, more repeatable control over exactly where fermentability locks in. Direct-fire and RIMS/HERMS systems can add this step easily by heating the mash in place; a simple cooler-based mash tun needs a calculated hot-water addition instead.