Back in the old days, a lot of us had to research this kinda stuff on our own.
That's how many people around here gained the knowledge they have assisting others.
Here's how we did that, whatever the ? at hand was we would look through the w3c specs.
And here it is, the specs for svg
And what you are asking about is right here
5.8. The ‘desc’ and ‘title’ elements
The ‘desc’ element represents more detailed textual information for the element such as a description. This is typically exposed to assistive technologies to provide more detailed information, such as a description of the visual appearance of a graphic or help to explain the functionality of a complex widget. It is not typically available to other users, so should not be used for essential instructions.
Authors may associate detailed information, including visible text, with part of the graphic using ‘aria-describedby’ attribute (on the described element or a parent container), with the value being an ID reference to one or more SVG or HTML elements containing the description. The ‘aria-describedby’ attribute takes precedence over the child ‘desc’ when providing a description. If an element has both visible description and a child ‘desc’ element providing supplementary information, authors should explicitly include the ‘id’ of the element itself in its own ‘aria-describedby’ list, in order to concatenate the two descriptions together.
The ‘title’ child element represents a short text alternative for the element.
On a link, this could be the title or a description of the target resource; on an image or drawing object, it could be a short description of the graphic; on interactive content, it could be a label for, or instructions for, use of the element; and so forth.
Authors should not provide redundant information in a ‘title’ element if there is also a visible label for the drawing element (e.g., using a ‘text’ element). Instead, the visual label should be associated with the drawing element using an ‘aria-labelledby’ attribute.
Interactive user agents should make the plain text content of ‘title’ elements available in response to user interaction, in a manner consistent with platform conventions; existing user agents commonly render ‘title’ elements as a tooltip on hovering the parent element.
Authors should provide a ‘title’ child element to the root svg element within a stand-alone SVG document. Since users often consult documents out of context, authors should provide context-rich titles. Thus, instead of a title such as "Introduction", which doesn't provide much contextual background, authors should supply a title such as "Introduction to Medieval Bee-Keeping" instead. For reasons of accessibility, user agents should always make the content of the ‘title’ child element to the root svg element available to users. However, this is typically done through other means than the tooltips used for nested SVG and graphics elements, e.g., by displaying in a browser tab.