A graph shows a relationship between two variables: independent and dependent. The dependent variable is in the y-axis, while the independent variable is in the x-axis. When it says, plot velocity against time, that means that velocity is in the y-axis in meters per second, and time is in the x-axis in seconds.
An example of a velocity vs time graph is shown in the attached picture. In this example, the motion is in constant acceleration. This is because it is linear, thereby, the slope is constant. If you want to find the instantaneous velocity at a certain time, create a vertical line from the x-axis until it reaches the diagonal line. Then, draw a horizontal line towards the y-axis to know the value of the velocity.
An example of a velocity vs time graph is shown in the attached picture. In this example, the motion is in constant acceleration. This is because it is linear, thereby, the slope is constant. If you want to find the instantaneous velocity at a certain time, create a vertical line from the x-axis until it reaches the diagonal line. Then, draw a horizontal line towards the y-axis to know the value of the velocity.