Jane Eyre explains her thoughts of presentiments, sympathies, and signs at the start of chapter 20:

Presentiments are strange things! and so are sympathies; and so are signs; and the three combined make one mystery to which humanity has not yet found the key.

Here, Jane introduces her superstitious beliefs. After, she elaborates on each of these three words.

I never laughed at presentiments in my life, because I have had strange ones of my own. (chapter 20)

Presentiments are predictions or premonitions about the future. Jane believes in presentiments because she has experienced moments where her gut feelings predicted future events.

Sympathies, I believe, exist (for instance, between far-distant, long-absent, wholly estranged relatives asserting, notwithstanding their alienation, the unity of the source to which each traces his origin) whose workings baffle mortal comprehension.

Sympathies, she explains, are common understandings or emotions between different individuals. There is an inexplicable connection (or sense of understanding) between some individuals, despite the distance between them:

And signs, for aught we know, may be but the sympathies of Nature with man.

By signs, Jane refers to events or objects that signal or predict a future event.

Jane's superstitious beliefs are shown throughout the novel. Some examples of signs, sympathies, and presentiments include the following:

1.) Early in chapter 20, Jane explains that dreams of children are a sign of trouble to come:

I one night heard Bessie Leaven say to Martha Abbot that she had been dreaming about a little child; and that to dream of children was a sure sign of trouble, either to one’s self or one’s kin. (chapter 20)

This dream is particularly concerning to Jane because she's been dreaming of children lately:

For during the past week scarcely a night had gone over my couch that had not brought with it a dream of an infant. (chapter 20)

2.) An example of sympathies is seen in chapter 22 when Jane explains how Mr. Rochester seems to understand her thoughts and feelings without Jane explaining them:

Mr. Rochester had sometimes read my unspoken thoughts with an acumen to me incomprehensible." (chapter 22)

These are two of many examples of presentiments, sympathies, and signs in Jane Eyre.

Jane mentions all three in the opening sentence of Chapter 21, but we see these in Chapter 20, too. In her use of Romantic conventions such as its emphasis on nature and the supernatural, Brontë creates a recurring feminized image in the form of the moon, which functions as a maternal figure providing guidance and protection, a tangible representation of power struggles in a patriarchal society, and a symbol of women’s strength. Thus, when Jane sees the moon “look” at her at the beginning of Chapter 22, she notices it is “beautiful, but too solemn,” for it is watching over her to protect her from mischance (a natural sign of maternal love that she lacked as a child) and a presentiment that mischance might soon happen, as it does when she hears the screams in the middle of the night, just as she rises “and stretch[es] [her] arm to draw the curtain.” In Chapter 27 we again see the moon as an important symbol. Here, in a moment of moral strife when Jane struggles to maintain her resolve to leave Thornfield after discovering Rochester’s wife, the moon appears as a moral guide, also offering sympathy, as if she knows the difficult decision Jane is facing: “She broke forth as never moon yet burst from cloud . . . then, not a moon, but a white human form shone in the azure . . . It whispered in my heart—‘My daughter, flee temptation.’” Then, in Chapter 21, Jane has a series of dreams about an infant. How do these dreams function in the story?