Let's look at your sentences and make them even better!
Your sentence: "Sunny meadows are reigned by grasses."
It's almost correct, but instead of "are reigned by," we say "are ruled by" or better yet, "are covered with" or "are full of." So, you can say: "Sunny meadows are covered with grasses."
Next sentence: "Their thick growth and nice smell attract a lot of animals."
This sentence is good! It tells us that the grasses grow thickly and smell nice, so many animals come there.
Last sentence: "The grass also provides cover and food for insects and birds."
This sentence is perfect! It explains that grass gives both shelter (cover) and food to insects and birds.
Putting it all together in a corrected and easy way:
"Sunny meadows are covered with grasses. Their thick growth and nice smell attract many animals. The grass also provides cover and food for insects and birds."
Great job! Keep practicing your sentences!