A CISA is a professional certificate that gives a bit more clout to the folks who are tasked with keeping an organisation's IT systems safe and secure. ISACA is the organisation that offers the CISA certificate, and it's pretty much focused on making sure the organisation isn't going to get hacked, and that they've got a grip on their IT risks and more.
And at the end, CISAs work to make sure the organisation's data is safe, and they are following all the rules they need to - and that their IT systems are just running smoothly.