Purport

In this Shloka Bhagvan Shri Krishna explains one of the core characteristics of Himself. Shri Krishna explains He is without beginning and without end and Lord of all worlds. Shri Krishna himself is the source of the whole universe, all deities, and other beings. Bhagvan Shri Krishna himself is God and supreme being is mentioned in almost all chapters except chapter 1 and in detail in chapter 7. But what is important in this Shloka is the use of two words birthless (ajam) and without end( anādiṃ).

Many times there arises confusion if Bhagvan Shri Krishna is birthless then what these incarnations as Bhavan Ram, Bhagvan Krishna, Bhagvan Bavana, Bhavan Parsuram, etc. Hadn't they all born?
The answer is, no Lord Ram, Lord Krishna, Lord Bavana, etc were not born, but Lord Vishnu appeared as a child in the womb. This process is called Incarnation. The very very meaning of Incarnation is when God appears in a certain form with own will and potency. To understand Incarnation fully, we should refer to other incarnations, such as the Incarnation of Shri Narsimha, Varaha, Matsya, Mohini, etc, In all these incarnations Bhagvan Vishnu simply appeared and disappeared without with being in the womb of a mother.
The process of Incarnation is very clearly defined in chapter 4 like this

Although I am birthless, imperishable and immutable and the lord of all living beings, yet by subduing my nature, I appear in this world in My original transcendental form by my divine potency.
--Ch4.Sh7

The process of Birth, on the other hand, is when a living being is born under the natural process of conception. A living being has no control over his birth nor does he alter or influence it in any way.

Lord Shri Krishna the supreme personality of Godhead is never born and never going to end. He is birthless and without end. This is one of the first things a devotee should comprehend about Shri Krishna