The Header tags are the most important thing in the current SEO market.
The H1 tag presents the Main heading to the search bot and users.
The H2 tag presents the secondary or sub heading to the bot and user.
and similarly the priority get on.
H1 tag is used for heading, whereas H2 tag is used for sub heading. The ALT tag is used to tell search engines what your website image is all about as Google can't read images.