The terms Township and Range, or just Range, refer to the United States Public Land Survey System[USGS en-wp] (PLSS), which was begun very shortly after the establishment of the United States to allow the government to manage lands west of the original 13 colonies. The system works by defining ‘zero points’, and numbering rectangles of land which are north or south, east or west of that zero point – an X, Y graph of the land.
Under the PLSS most of the land is divided into 6 mile square Townships, which are further subdivided into 36 square mile “sections”, which can be further subdivided into quarter sections, quarter quarter sections, or irregular government lots. Markers are placed at each corner of a section, and at quarter sections, called monuments. Modern monuments are usually inscribed tablets set on metal rods or in concrete, but less permanent markers have been used in the past and may make finding the monument more challenging for surveyors!
And that is an important point about the PLSS: most land documents reference the PLSS, making it ‘the law’ about exact dimensions of a given titled piece of land. A surveyor can show you where a piece of land is relative to the local PLSS monuments.
However, even though we now know how to turn T155N – R49W into “the township 155 north of the base line and 49 west of the prime meridian”, this is not easily translated into a spot on the map. Fortunately, there are a lot of township and range converters online these days. Depending upon the specific tool you look at, they might return the latitude and longitude coordinates, or display a map of the location, or build an embeddable map html for you. The important thing is: they turn something a bit obscure into a useful way to find what you are looking for.
Hopefully this will help you in your Marshall County genealogy research!