Eighth notes look similar to quarter notes, except they are connected by a beam. It takes a group of two eighth notes beamed together to equal one quarter note in the 4/4, 3/4, and 2/4 time signatures. That means this group of eighth notes must sound evenly over 1 beat.
Many times when you see two eighth notes together, they are connected by a beam. This beam is a darkened line that runs horizontally (left to right) to connect the tops of the stems.