I guess 'Stupa' is the word I first learned in Laos, to denote a structure built to shelter/house/protect relics of some holy or revered person. 'Chedi' seems to be the Northern Thai equivalent. 'That' is another title for such a structure - pronounced somewhat /t^t/
(tut). The plaque gives the date of this as 1487. Tthe archway looks like it had a figure built within it. Some of the plaster is still there.