How Do I Analyze Functions with Master Theorem to Determine Big-O

I've been struggling with understanding Master Theorem for a while now. Most of the info I find seems to assume that I understand certain vocabulary. So here comes a very long question...

Consider the following examples:

const items = [1, 2, 3, 4, 5];

// O(1) Constant Time
const getByIdx = idx => items[idx];
}

// O(n) Linear Time
const findItem = val => {
  for (let i = 0; i < items.length; i++)
    if (items[i] === val) {
      return items[i];
    }
};

// O(n^2) Quadratic Time
const matrix = [
  [1, 2, 3],
  [4, 5, 6],
  [7, 8, 9]
];

const findItemInMatrix = val => {
  for (let i = 0; i < matrix.length; i++) {
    for (let j = 0; j < matrix[i].length; j++) {
      if (matrix[i][j] === val) {
        return matrix[i][j]
      }
    }
  }
};

// O(log n) Logarithmic Time
const arr = [1,2,3,4,5,6,7,8,9,10];

const binarySearch = (num, data) => {
  if (data.length === 1) {
    return data[0];
  }

  let low = data.splice(0, Math.ceil(data.length / 2));

  if (num > low[low.length - 1]) {
    return binarySearch(num, data);
  } else {
    return binarySearch(num, low);
  }
}

The examples above are simple enough to determine Big-O without the formula, but I'm curious as to how to use the formula instead:

According to WIkipedia:

 procedure p( input x of size n ):
   if n < some constant k:
     Solve x directly without recursion
   else:
     Create a subproblems of x, each having size n/b
     Call procedure p recursively on each subproblem
     Combine the results from the subproblems

Here's where I get lost:

What is a subproblem? What lines would be subproblems in my code?
"subproblem" is vague to me. Would a subproblem be...
A statement? return items[i]
An expression? return items[i] + 1
A loop?

What do I plug in for b?
What is b? What piece of code would represent it?

What do I plug in for a?
What is a ? What piece of code would represent it?

What do I plug in for k?
What is k ? What piece of code would represent it?

How would I use the formula on these examples?
Is anyone willing to work through an example using the code samples above in the formula?

Thanks for your help!