I've been reading about partial orders and partially ordered sets and have come across sentences like "Suppose that $\preceq$ is a partial order on $X$" and "If $x\preceq y$ and $y \preceq z$ then $x \preceq z$ for $x,y,z \in X$".
How do you pronounce the symbol $\preceq$ in these cases?
It depends on the context.
If there is no ambiguity, "less than or equal to" works.
In a lecture, you might pronounce it "curly less than" to help people who are taking notes. If you want a short way to pronounce it, you might vocally label it "r" or "rel" (short for "relation"), as in "Suppose that rel is a partial order", but this is less standard and I would personally say "Suppose that curly-less-than is a partial order; I will call it 'rel' from now on".