You might be familiar with the expression terms of crescendo, diminuendo, forte and piano, as well as mezzo forte, mezzo piano, fortissimo and pianissimo, if not then here is their definitions:
Crescendo – gradually becoming louder
Diminuendo – gradually becoming softer
Forte – loud
Piano – soft
Mezzo Forte – moderately loud
Mezzo Piano – moderately soft
Fortissimo – very loud
Pianissimo – very soft
The difference between the top two on the list, crescendo and diminuendo and the others that are listed is the word gradually. This one word makes such a difference and can make an enormous impact on the overall sound of the piece of music.
When playing forte, piano, mezzo forte, mezzo piano or any other indication of what the volume of the music should be, the change should be immediate. For instance, if you were playing a passage of music that was marked as mezzo piano and then it changed to forte, then a clearly distinct change in the volume should not only be evident to yourself, but to anyone else that was listening too.
For some piano players, the gradual change of a crescendo and a diminuendo can be a challenge to make happen. This can be compounded by the fact that sometimes you can have only as little as 4 notes to gradually change from one volume to another. Sometimes you can be lucky and have several bars to make the change.
Taking the situation of having a few bars to do a crescendo or a diminuendo, I like to think of the change of volume is akin to the change in volume of a radio when you turn a dial slowly moving it either clockwise or anti-clockwise (as which ever way is appropriate). It is like every note is minuscule louder (for crescendo) or softer (for diminuendo) than the previous one until the desired end result is achieved. In the circumstance where you might have to do a crescendo over only 4 notes, then the way I think of it as each note is a quarter of the way between the starting and the ending volume. Likewise if you had only 6 notes to gradually go from forte to piano, I would try and get each note approximately a sixth quieter than the previous note.
Over the years I have seen and helped piano students that do not find it easy to play crescendo and diminuendo easy to master. I hope that these insights, along with practice will help you to play crescendos and diminuendos easily and effectively.