An A record is one of the most basic DNS records.
This takes your domain (yoursite.com) or subdomain (www.yoursite.com) and points it at an IP address (usually your servers IP address).
There can be many different A records. For example, foo.yoursite.com could point at one server and bar.yoursite.com could point at a completely different server.
Do not change A records unless you know what you are doing.
Changes to A records take time to propagate over the internet.
If you make a change and then realise you’ve done it wrong, you could be in for a 24-hour wait until it’s fixed.