According to the dictionary that comes with Chi, it's "a vacant lot where children play". Who brings the sand?
I assumed Linda's question was rhetorical. She knows she can click on a word for its definition ... or even use a search engine to learn about a word .... right, Linda? .... right??
I think of a sandlot as a place in a subdivision which has not yet had any construction — where there isn't any real grass, just a bunch of weeds and typically used by kids in the area for playing (unorganised) sports, especially baseball and baseball-type games (softball, stickball, 500, etc.). The field isn't necessarily sandy unless the normal 'soil' of the area is sandy. I would guess the name might derive from having an empty lot used by the subdivision developer (who typically owns the land until it is sold to a new home buyer) as a place to store building materials, including sand (for concrete) for construction being done at nearby lots.