People have to start from the ground, then first step, second, third and so on to reach their destination floor. Exactly the same way students have to start from Kindergarten, then grade one, grade two and three and so on to reach their math destination. Also, if some of the steps are broken in the staircase, it is still hard to reach the desired floor using those steps. Same way, if you are missing some of the basic concepts from elementary grades, math for you is still hard. Now, the kindergarten, first grade and second grade are like first couple of the steps of the stairs. You can learn this level of math easily, as you can jump enough to take yourself to second or third step of the stairs easily. As it is very hard to reach sixth or seventh step of a stairs by jumping from the ground, exactly the same way to learn grade five or higher grade math is very hard (or impossible) without having the good knowledge of the kindergarten to grade three or grade four math. Now, consider one person is jumping on the ground to reach the third floor of a building. Can this person make it? Never, if he is not Spiderman. For this person, to reach the third floor by jumping is impossible or very hard and finally he gave up saying that it was very hard to reach third floor.
As a math tutor for 15 years, I have tutored hundreds of students from all the grade levels. I found most of the students were not comfortable with mathematics. More than 80% of them said, "the mathematics is very hard." Why do most of the students find mathematics a hard subject and try to avoid it? The answer for the above question is hidden in a simple example. I always give the example of stairs to my students, and giving the same example in this article. I compare the steps of a staircase to the concepts in mathematics. As this is very hard to reach higher floors of a building without stairs (or elevators these days), same way learn higher concepts in mathematics without learning basic concepts is very hard.