What is search intent?
Search intent identifies the OBJECTIVE behind a search query. Search engines try to determine the WHY of the query of the person who is searching, they also look at his past search queries to determine the best results.
For example, A person is searching "Cost of OnePlus Nord", this could be a transactional keyword that has the potential to end up in a purchase, also this could be an informational keyword, ideally for research purpose. Based on the past and current search behavior of the user, the search engine delivers the best result. If the intention is a purchase, a person might have also used the queries "Best smartphones to buy", "best-branded smartphones to buy". If the intention is informational, a person might have also used the queries "Cost comparison of smartphones", "compare the cost of upcoming smartphones" etc. Search engines can understand this user behavior and deliver personalized search results.
Why should you be optimizing for it?
Back in 2018, Google updated the algorithm to include Neural Networks. This was a game-changer for SEO optimization. A neural network is a series of algorithms that recognize underlying relationships between a set of data through a process that emulates the way the human brain operates. This helps the search engine to understand the user intent better, and they expect webmasters to optimize the content best fit for the intent of the search query. The BERT algorithm update enhanced this process.
Also, Frédéric Dubut of Bing told that SEOs should focus on intent research over keyword research in 2020. The same opinion was shared by John Mueller of Google as well, he seems to agree that search engines will continue to get better at understanding intent and rely less on keywords.