A subdomain is a secondary domain that is part of a larger primary domain. It is created by adding a prefix to the primary domain name, such as 'blog' or 'store' (e.g., blog.example.com or store.example.com). Subdomains can be used to separate different sections or categories of a website, or to host content that is distinct from the main site.

Subdomains can affect a site's SEO in a number of ways:

1. Domain authority: Subdomains are treated as separate entities by search engines, so a subdomain will not inherit the domain authority of the primary domain. This means that a subdomain may have a lower search engine ranking than the primary domain, and it may be more difficult for the subdomain to rank well in search results.
2. Link equity: Links to a subdomain do not pass as much 'link equity' (ranking power) to the subdomain as links to the primary domain. This means that it may be more difficult for a subdomain to rank well in search results if it relies on external links for its ranking power.
3. Keyword cannibalization: If a website uses subdomains to host content that is similar to the content on the main site, it can lead to keyword cannibalization, where the site's pages compete with each other for ranking in search results. This can dilute the ranking power of the site's pages and make it more difficult for any one page to rank well.
4. Crawling and indexing: Subdomains may be treated as separate websites by search engines, which means they may be crawled and indexed separately from the main domain. This can lead to duplication of content and can dilute the ranking power of the site's pages.
5. Brand confusion: Subdomains can create confusion for users and may not be perceived as being part of the same website as the main domain. This can lead to a lack of trust and may discourage users from engaging with the site.
6. Link building: It can be more difficult to build high-quality links to a subdomain, as external websites may be less likely to link to a subdomain rather than the main domain. This can make it more difficult for the subdomain to rank well in search results.
7. User experience: If a website uses subdomains to separate different sections or categories of the site, it can be confusing for users to navigate between the different subdomains. This can lead to a poor user experience and may discourage users from engaging with the site.

Overall, it is generally better for SEO to host content on subdirectories of the primary domain rather than on subdomains, unless there is a specific reason to use a subdomain (e.g., to host content that is completely distinct from the main site). Hence, there may be valid reasons to use subdomains in some cases, it is generally best to avoid them if possible, as they can have negative impacts on a site's SEO.