The Export panel writes seven formats. You choose one under Format.

The formats
PDF	The one for printing somewhere else, or for sending. Texts, barcodes and shapes stay sharp at any size, and a PDF can hold all your pages in one file. What exactly goes inside is up to Contents: see Vector output modes.
JPEG	A picture that stays small, on a white background. Best for photos; a barcode can come out slightly blurred at its edges.
PNG	A sharp picture, with a transparent background where your label has none. Best for putting a label or an element into another document or a web page.
WEBP	Like PNG, transparent background included, in smaller files - for the web.
GIF	An older picture format, on a white background. For programs that can't read the others.
TIFF	A sharp picture with a transparent background, the one some print and publishing programs ask for.
BMP	A plain, uncompressed picture on a white background: big files, readable by almost anything.

One file or many. A picture holds one page, one label or one element, so exporting several makes several files, numbered. A PDF can do either: see Export as in the Export panel.

Resolution. Image resolution sets how many dots per inch a picture has: 96 dpi looks right on screen, 300 dpi or more is for printing. A Letter page at 300 dpi is about 2,550 by 3,300 dots, so the files grow fast. In a PDF it applies to the pictures inside, and to everything when Contents is Image.

Want just a quick picture of one element, a label or a page? Right-click it and choose Copy as image or Save as image...: that's a single picture, at screen resolution, in any of the picture formats above.