In the video above, the character reaches for a clip, reloads and then returns to an idle pose.
While this is perfectly fine to use as a reload animation, a Montage will provide much more control.
For example, suppose you wanted the character to reload a variable amount of bullets instead always the same amount.
By splitting this animation up into three parts (grabbing a clip, reloading, and returning to idle) and combining those parts in a Montage, you could section them off and selectively play any or all of those sections.
Below is an example Montage used to handle the reloading of a shotgun, sectioned it off into three parts: Start, Loop, and End.
With Blueprint or C++, we can define that when the player presses a button, the animation is played, beginning at the Start section.
The looping middle section, called Loop, immediately follows the Start section and is set within the Montage to repeat indefinitely.
If the player releases the reload button, or they've filled the shotgun with bullets, the animation will stop looping and can be set to jump to the End section to transition out of the animation as seen in the video below.