What does this part in code mean? (Java)

Hey guys im new to java and i came across this thing Ok so here i got something i just cant comprehend

here i have this class:

class FailSoftArray {
private int a[]; // reference to array
private int errval; 
public int length; 


public FailSoftArray(int size, int errv) {
    a = new int[size];
    errval = errv;
    length = size;
}

public int get(int index) {
    if(indexOK(index)) return a[index];
    return errval;
}


public boolean put(int index, int val) {
if(indexOK(index)) {
a[index] = val;
return true;
}
return false;
}

private boolean indexOK(int index) {
if(index >= 0 & index < length) return true;
return false;
}
}

What does indexOK(index) mean? What does it do?