SILE, abbreviated for Simon's Improved Layout Engine, is a typesetting system, whose job is to produce beautiful printed documents from raw content. It reads input from its scripting language or XML which describes what you want to happen so that the SILE engine will make certain formatting decisions about the best way for those instructions in the input to be turned. SILE is basically a modern rewrite of TeX. It'is written in Lua, so it is very easy to extend or alter the behaviour of the SILE typesetter. For example, in SILE, you can alter the behaviour of the typesetter and write a very short add-on package to enable grid typesetting. For the ecosystem and community support, TeX will remain streets ahead of SILE for some time to come. But in terms of core capabilities, SILE is already certainly equivalent to, if not somewhat more advanced than, TeX.