You may not have noticed this, but there are basic units of time, distance, and mass.
What this means exactly is best left to physicists, but for the rest of us: If you had a very (insanely accurate) watch, and you wanted to set it to exactly noon, not a nanosecond less or a femtosecond more, down the scale, there comes a point at which time is indivisible.
Same with length. The smallest unit of length is about 20 orders of magnitude smaller than a proton, which is already pretty small. This means, among other things, that there is no way to divide this smallest length.
It would be like dividing a photon, which you simply cannot do.
The smallest time is called the Planck time, and the smallest distance, the Planck distance. They were both named after Max Planck the famous physicist. The Planck time is how long it would take light to travel the Plank distance. This is an unimaginably short interval equal to a decimal fraction of 1 second, with 43 zeroes followed by a 1.
Since this is the shortest amount of time, that's where the big bang starts.
Books describe the Planck length and Planck time as the limits of what makes sense. Anything smaller than the Planck cannot be defined.
Then there's the Planck mass, which is about equal to a black hole with a radius of the Planck length.
Now, if there are smallest units of time, distance and mass, shouldn't there be corresponding maximum limits of the same things? There might be, but the values are of no value.
Physicists use Planck time, distance and mass, in quantum mechanics.
You might say they walk the Planck.