The main idea is to have every line of text fall on an evenly spaced grid. Here's an example where the middle paragraph is not grid aligned: https://i.stack.imgur.com/XskJu.png
Generally you also want the baseline of titles to fall on the grid. Possibly also the formulas, figures etc.
LaTeX is pretty bad at this: by default it inserts stretchable vertical space between paragraphs, and around things like bullet lists, centered text and formulas.
Stretchable space is a good thing when you have a lot of elements beside simple paragraphs: it gives TeX flexibility to produce a nice page layout. For example the optimal spacing around equations might not be a multiple of the inter-line space. And you might still want to have the last line land precisely at the bottom of the page, so there must be a stretchable space somewhere. This flexibility also helps avoiding widows and orphans[1].
On the other hand I think non-grid-aligned text looks terrible for novels, especially when the page is thin and you can see through the paper the text on the other side (it's much less noticeable if the lines on both sides are perfectly aligned). Grid typesetting is also nice when you have multiple columns of text on the same page: it looks odd if the lines of one column are not aligned with those in the next column.
Generally you also want the baseline of titles to fall on the grid. Possibly also the formulas, figures etc.
LaTeX is pretty bad at this: by default it inserts stretchable vertical space between paragraphs, and around things like bullet lists, centered text and formulas.
Stretchable space is a good thing when you have a lot of elements beside simple paragraphs: it gives TeX flexibility to produce a nice page layout. For example the optimal spacing around equations might not be a multiple of the inter-line space. And you might still want to have the last line land precisely at the bottom of the page, so there must be a stretchable space somewhere. This flexibility also helps avoiding widows and orphans[1].
On the other hand I think non-grid-aligned text looks terrible for novels, especially when the page is thin and you can see through the paper the text on the other side (it's much less noticeable if the lines on both sides are perfectly aligned). Grid typesetting is also nice when you have multiple columns of text on the same page: it looks odd if the lines of one column are not aligned with those in the next column.
[1] https://en.wikipedia.org/wiki/Widows_and_orphans