What it means
Alt text is what somebody hears instead of seeing the picture. It should describe what is in the image and why it is there, in a sentence, without starting with the words image of.
Why it matters to a small business
Two reasons, and the accessibility one comes first: a customer using a screen reader gets nothing at all from your menu photographed as a picture. The second is that images are opaque to search engines and assistants, so on an image-heavy site the alt text is a good share of the only text there is. Marketeer sends alt text through to Instagram, Facebook, LinkedIn and X when it publishes for you.
The bit people get wrong
A menu, a price list or an offer set entirely in a graphic is invisible twice over: to assistive technology and to every machine that might have quoted your prices. Put the words on the page as words, and use the image to make them look good.