Let's trace this for loop iteration by iteration, the way JavaScript actually runs it.
let i = 1; runs exactly once, before anything else i now holds 1. i <= 5 is checked: true, so the loop body runs, appending "Count: 1" to output. Then i++ runs, making i become 2.
The condition is checked again: i <= 5 is still true (2 <= 5), so the body runs again, appending "Count: 2". This cycle check condition, run body, increment repeats for i = 3, 4, and 5.
When i becomes 6, the condition i <= 5 finally evaluates to false, and the loop stops immediately, without running its body a sixth time this is exactly why the loop produces five lines of output (1 through 5), not six.
output += `<p>Count: ${i}</p>`; deserves attention too: += appends to the existing string rather than replacing it, which is exactly how output accumulates all five lines before being displayed at once, after the loop finishes.