Pronouns replace nouns. They are usually smaller words and take the place of a noun in sentences when the noun would be repeated too many times.
For example, consider reading a sentence like this one:
“Mary had a little lamb. The fleece of Mary’s lamb was as white as snow. Every where Mary went, Mary's lamb followed Mary. Mary's lamb even followed Mary to school once. Mary's lamb made other children laugh and play.”
Now, that might remind you of a kindergarten story but not in a pleasing way.
The structuring used in the above sentences is bad. There are two nouns here: “Mary” and “lamb”. Every time a reference to either is made, there is an awkward repeating of the nouns. Instead, they can be replaced with the smaller reference words.
Let’s re-phrase the paragraph now:
“Mary had a little lamb. It's fleece was white as snow. Every where she went, it followed her. It even followed her to school once. It made other children laugh and play.”
The paragraph isn't exactly like the Nursery Rhyme but it is shorter, easier to read and not as annoying as it was before. It's still not perfect, you can mix using nouns and a pronoun in a paragraph to give some variety at times.
Common Examples: he she you they themselves some it
Mira is speaking to her. (another lady mentioned as “her”) Sarah and Tina joined them. (another group) Mira is speaking to herself. (like talking in one’s head; thinking) Tina and Sarah are talking between themselves. (with each other) Tina and Sarah are talking to themselves. (both are talking in their heads; thinking)
Without these, we would have to use too many nouns, making sentence meanings muddy at times. Making regular use of them can make sentences clearer and shorter.