What does the part a[a[i-1] of this for loop mean?

It's nothing fancy. It's just a simple for loop with some compound statements. let me explain every bit of it.

The syntax ( writing sequence ) of for loop is below.

for(initial value for loop ; condition to check on every loop ; increment in initial value)
{
  body of loop
}

First Line :

int[] a = new int[6];

This line is only declaring an INTEGER ARRAY of length 6

Second Line :

for(int i=0; i<=10 ; i=i+2) a[i/2] = i < 5 ? 5 - i : 0 ;

This is a compound statement, First simplify this.

The above CODE can be written as

for (int i=0 ; i<=10 ;i=i+2)
{
 if(i < 5)
  {
     a[i/2] = 5 - i;
  }
  else
  {
    a[i/2] = 0;
  }
}

Third Line :

for(int i=a.length ; i > 0 ; i--) a[i-1] = a[a[i-1]] + i ;

The above code can be written as followed

for(int i = a.length ; i > 0 ; i-- )
{ 
  //decomposing a[a[i-1]]

  //lets assume that **i = 2** and **a[1] = 99**;
  a[ value of a[i-1] which is a[2-1] => a[1] which is 99 ];
 // so after putting value of a[1] the above equ will be

  a[99]
  //this is also be equal to some value, let say a[99] == 40;
  //then 
  
  a[i-1] = 40 + i;

  
  
}