Can you link me to exactly what you are talking about so I am sure to study the correct thing?
hiragana is the alphabet standard. Think of it as writing cursive, except you would write in cursive all the time
katakana is a stylized alphabet. Think of it as writing typeface except you would only use it for people's names, cool things, and foreign words
Kanji is the pictograph. Every word can be written in any three ways (for the most part)
Kanji is like....classic words...core fundamental words. Rice, cow, animal, place, house, etc. There are very few kanji for newer things. That is where katakana comes in.
When a kanji is used for many words, there are often endings done in hiragana. liiike eating is "taberu", which is also "tabemasu". In kanji, the symbol only stands for the sound "tabe", because as you can see, endings change to say different things. There is:
As you can see the only core part of the word that never changes is tabe...therefore, because kanji doesn't change, it only takes the place of tabe. Then you add the endings with cursive, so you get: