A records are the most common type of record type in DNS configurations. Many other types of DNS records will not work if you have not already set up A records (or AAAA records).
The A stands for address, which is fitting since an A record maps to point domains or subdomains to an IP address (IPv4). The values for A records, therefore, are always IPv4 addresses.
To provide redundancy, you can use more than one A record for the same domain. (Here, you need to add another A record with the same name, but with a different IP address for the value.) Or you can have multiple domain names point to the same IP address. (In this case, each domain name would have its own record pointing to the same IP address.)
One type of A record that is often used is an at-A-record (or @ A record), which is also called a root record. You will likely use this as your default A record. This record basically states the IP address for your default server. When you create an at-A-record, you use the @ symbol, and then you point the @ symbol at the IP address for your default server.