As @Flaw mentioned,
さえ is similar to
も meaning "even" (it can be combined with other particles as well). However, it carries the nuance of referring to a/the most "basic" or "obvious" thing in the context. Here are some examples:
先週の病気は本当に辛【つら】かった。水さえ飲めなかった。 → "My illness last week was really bad. I couldn't even drink water!" When one is sick, you expect them not to be able to drink alcohol, soda, etc. But water should be OK for most people. Even water, the most basic thing was intolerable for this person.
えりこは親友の花子にさえ知らせずに外国へ旅立った。 → "Eriko left on a trip to a foreign country without telling even her best friend Hanako." You'd expect her to not tell her co-workers, acquaintances, etc. But best friends usually tell each other everything, and she didn't tell even her.
山の上には夏でさえ雪が残っている。 → "Even if it's summer, there is snow left on the top of the mountain." You expect snow to be on top of a mountain in winter, and some leftover in the spring. But usually by summer it's all gone, but for this mountain the snow remains even then.