An A record is a DNS record that points a name at an IPv4 address: the numeric address of the server that answers for it. It is most often used for the bare domain, yourbusiness.co.uk without the www, to send visitors to the server hosting the website.
How it is used
An A record has a name, such as @ (meaning the bare domain) or www, and an IP address as its value. Its newer sibling, the AAAA record, does the same job for IPv6 addresses.
The bare domain normally has to use an A record rather than a CNAME, because the bare domain also carries other records, such as the email ones, and a CNAME cannot share a name with anything else. Some DNS providers offer a workaround, often called ALIAS, ANAME or CNAME flattening.
Why it matters to a business
The A record is usually one of the two records that change when a website moves to a new host; the other is the www record. Use the exact values the new host gives you rather than an address copied from an old guide, because hosts do change them.
Changing the A record does not affect email, which is routed by MX records. Delete or overwrite the wrong record, though, and it can.