In terms of internet, spider refers to BOT or the web crawler which is basically an automated computer program that takes in an order and executes it just like a Robot. The spider crawls over the different web pages and captures the content that is there on the page. It stores a copy of the page and helps the search engine by providing the information. When a user searches for something through the search engine then the search engine processes the data that has been stored by the spider and index it which helps it to provide the user with the required data at a faster speed. Thus the spider or the web crawler essentially helps a search engine to come up with the relevant data.
Some of the people also use the spiders to get crawl over different web pages and get them specific information. For instance, the web spider can be used for getting email addresses that can then be used for email marketing and is also used for spamming. The web crawlers also look out for new changes in the already crawled websites and monitors then to note for the changes and provide with the most relevant search results. Getting your website spidered is therefore important to show up in the SERP.