Plugin allow you to extend core of wordpress. Most popular used plugin are-
Show Custom form on website
Contact Form 7 : https://wordpress.org/plugins/contact-form-7/
For SEO
WordPress SEO By Youst : https://wordpress.org/plugins/wordpress-seo/
For Performance
WP Super Cache : https://wordpress.org/plugins/wp-super-cache/
For E-commerce
Woo Commerce : https://wordpress.org/plugins/woocommerce/
For Easy Customization of page content
Advanced Custom Fields (ACF):https://wordpress.org/plugins/advanced-custom-fields/
Security
Captcha : https://wordpress.org/plugins/captcha/
I will keep posting on important wordpress plugins. keep looking.