8th notes are exactly twice as fast as 1/4 notes, therefore you can play 2 x 8th notes inside every 1/4 note count. This is what an 8th note looks like on its own..

You'll notice that the 8th note has 1 flag falling of its stem. You'll also see that the 8th note rest (next to each 8th note) also has 1 flag falling of its stem. This is how you can recognize notes with their equivalent rests, they always share the same number of flags/stems.

If we were to write 8th notes next to each other, we wouldn't write a stem and flag for each one, we'd save time by just joining all the 8th notes together with a single beam. Again, because an 8th note and an 8th note rest only have 1 flag, 8th notes that are joined together must also only have 1 beam..