An isohyet is a line on a map delineating a change in rainfall. Just as topographic maps have contour lines (aka an isohypse), a rainfall map has isohyets.
You are most likely to hear about some very specific isohyets; a 250mm isohyet (aka 10-inch isohyet) is the point at which there is enough annual rainfall to support dry farming; somewhere around a 380mm isohyet (or 15-inch; oft cited as important in China) to 500 mm (or 20-inch; oft cited to be important in the American Great Plains) is the point at which farming is generally held to be a fine and productive enterprise.
In geography you will sometimes see different climates defined by their isohyets. While the exact numbers vary greatly (as in, by hundreds of mm), a common division is that arid climates lie below the <250mm isohyet, semiarid fall between 250-400mm, subhumid 400-1000mm, and humid from 1000mm on up. However, it is more common to define these terms in relation to their ratio of rainfall to evaporation than in strict terms of rainfall.
In tracking storms you will often see references to 24-hour isohyets, but rarely will you see the term used when referring to smaller increments; your meteorologist might show hourly or 15-minute updates on rainfall, but the maps they use are more likely picturing isobars than isohyets.