Many camera lens lash-up have a mechanical stop that prevents the lens from racking forward beyond a certain distance. This sets the minimum focus distance to about 2 feet (600mm). General purpose camera lenses are optimized for nearby and far subject distances and slightly compromised when tasked to work close-in.
Additionally, the f-numbers engraved on the lens barrel become invalid when tasked to work in close. The f-number is derived by dividing the focal length of the lens by the working aperture diameter. The engraved values are only correct when the camera is imaging objects at infinity (as far as the eye can see ∞). As you task the camera to work closer and closer, the physical distance, lens to image plane is elongated. At unity (magnification 1 often stated as 1:1), the lens is racked out 2X the focal length. Thus a 50mm tasked to work at unity performs as a 100mm lens. The engraved f-numbers thus are two f-stops in error. This is called “bellows factor”. The formula to correct is (M+1)^2 (magnification + 1 squared). Thus at unity (magnification 1) the math is (1 + 1) ^ 2 = 4. This value 4, is a multiplier, you correct for underexposure by multiplying the exposure time by 4 or opening up the aperture 2 f-stops.
For this reason, it is industry practice to stop the forward racking of the lens when the error reaches 1/3 f-stop. All this reasoning is moot if the camera has through-the-lens metering. Such a design reads the exposure under the influence of the bellows factor error. Thus the need for the photographer to make a manual compensation is avoided. This is why many modern cameras do not impose a close focus limitation.
One more thing: General lenses are designed to work a curved world (3 dimensional) and project this image on a flat image sensor or film. When working extremely close, subjects are generally shallow and this fact often results in some sharpness compromise. The countermeasure is to invert the lens as the rear elements are optimized to work a flat surface.
A macro lens is optimized to work in close, compromised when tasked to work afar. Its design is a seamless countermeasure that eliminates the bellows factor.
You can work in close with a general lens by spacing it further than its mechanical limiting stops. We use spacers called rings or tubes. If you do, you must manually take into account bellows factor unless the camera meter reads through the lens. You can also use close-up supplemental lens to work in moderately close (no bellows factor for this lash-up).