The alt and title attributes of an image are commonly referred to as alt tag or alt text and title tag even though they’re not technically tags. The alt text describes what’s on the image and the function of the image on the page. So if you have an image that’s used as a button to buy product X, the alt text would say: “button to buy product X”.
The alt tag is used by screen readers, the browsers used by blind and visually impaired people, to tell them what is on the image. The title attribute is shown as a tooltip when you hover over the element, so in case of an image button, the button could contain an extra call-to-action, like “Buy product X now for $19!”.
Each image should have an alt text. Not just for SEO purposes but also because blind and visually impaired people otherwise won’t know what the image is for. A title attribute is not required. It can be useful but in most cases, leaving it out shouldn’t be much of an issue.