Emoji usually represent an emotion, object, person, symbol or idea. A face emoji can show tone, a heart can show affection, and a symbol can make a message easier to scan.
Meaning is not always fixed. 😭 Loudly crying face can show real sadness, exaggerated emotion or laughing so hard that someone is crying. 💀 Skull can mean death, shock or "I am dead" from laughter.
The best way to read an emoji is to look at the whole message. The same emoji can feel warm, sarcastic, playful or serious depending on the words around it, the relationship between people and the place where it is posted.
For a broader starting point, see what is an emoji and emoji meanings by context.