Two ways to get a transparent picture, and when each fits.
Generating with transparency
Asking for a transparent background up front works best for objects, icons, stickers and products that will sit on top of something else. The result has no background at all, so it can go on any colour or picture. In Melon Cut Design it is an option when you make a picture.
Removing a background afterwards
When you already have the photo - a product, a portrait, a logo on white - Remove background isolates the subject and makes everything else transparent. Edits use 2 to 5 credits, depending on the tool.
Which file to keep
Transparency survives only in formats that support it, such as PNG; a JPEG fills the transparent area with a colour. When a website or printer asks for a transparent PNG, that is the file to send.