What is the meaning tiny bundle of energy smaller than an atom At the beginning of a video on youtube titled "history of the world in 2 hours", it is said that "every thing begins with this tiny bundle of energy smaller than an atom". I want to know what it means.  atom is matter.  energy is different from matter and is measured in joules where as size is measured in volume units. How can one say energy smaller than an atom?
What the video is clumsily alluding to is that the universe was very much smaller than it is today. However, if the universe is infinite, then it was still infinite at all times in the past. All one can say is that the density was much higher.

Sometimes commentators are more precise. One can define a size scale for the currently observable universe and ask how that volume of space evolved with time. This of course assumes an accurate knowledge of cosmology and the relevant cosmological parameters.

A ball park number (see here) is that the universe has got bigger by a factor of $10^{30}$ since the end of the inflationary epoch (at $t \sim 10^{-32}$ s; which is not fully understood yet). Thus the current observable universe, of size $10^{27}$ m, was contained within a diameter of 1 mm at the end of the inflationary epoch. Prior to the inflationary epoch it must have been compacted into a much smaller volume than this - and I think this is the origin of the statement in your question.

The reason that your source talks about energy is because that is in what form the density of the universe was at that epoch. It was only later that this energy started to "freeze out" into various forms of matter. It is now commonly supposed that the Higgs mechanism or some form of it, is what gives elementary particles the property of a rest mass that might serve as a definition of "matter". The Higgs mechanism kicks in at the epoch of electroweak symmetry breaking (at temperatures of about $10^{15}$ K), giving paticles mass. Prior to this the density of the universe was in the form of massless entities that you might refer to as "energy". This symmetry breaking occurs after the end of the inflationary epoch at $t \sim 10^{-12}$s, and therefore when the currently observable universe was larger than 1 mm. So prior to this, and certainly when the currently observable universe was "smaller than an atom", the contents of the universe could be described in term of energy (without rest mass).