Boxes are the basis of all (low level) TeX typesetting. As the building blocks they store typeset parts of lines, paragraphs or pages. For storing material not being typeset yet, like input text for later use, use [[Buffers_in_LuaTeX|Buffers]].
The introduction to LuaTeX boxes can be found in [https://www.overleaf.com/learn/latex/Articles/Pandora%E2%80%99s_%5Chbox:_Using_LuaTeX_to_Lift_the_Lid_of_TeX_Boxes|Pandora’s \hbox: Using LuaTeX to Lift the Lid of TeX Boxes].
LuaMetaTeX boxes serve the same purpose, but their properties are changed and extended. For the details see [http://distribution.contextgarden.net/current/context/latest/doc/context/documents/general/manuals/lowlevel-boxes.pdf Low Level TeX: Boxes] manual.