A Shifu is technically a Master in something.
It is not only applied to Martial Arts either.
When someone masters an art, trade or skill, they can be considered a Shifu.
That is, they have achieved the highest form of a certain skill.
Being a Shifu generally also means you have enough knowledge to teach others in the art, trade or skill. However, not every ...
My understanding is that "eat bitter" is perseverance through hardship. There are old Chinese idioms that are 4 characters, like "eat bitter taste sweet" meaning something like suffering now brings rewards later - in this case the suffering is hard training and the reward is gongfu.
There's another one like "not bitter not tea" ...