"Shabad" means sound, "Guru" means teacher or knowledge that transforms you. The simplest meaning of Shabad Guru is a special sound that is a teacher. But what is it that we learn from the Shabad Guru and how does the use of sound act as a teacher?
To answer this question look at a deeper definition of Shabad Guru from its root structure. "Shabad" comes from Sha- and -bd. Sha means the expression of the ego, the attachments we identify with. Bd means to cut out/off or to eradicate. The root meaning of Sha-bd is that which cuts the ego. It is not just any sound. It is not just a wise sound or a song of truth. It is a sound that cuts away the ego that occludes the truth from you.
Ego is formed by the mind's constant automatic processing that attaches us to and identities us with objects, feelings and thoughts. The ego is not inherently bad. It is just limited.
ਹਊਮੈ ਦੀਰਘੁ ਰੋਗ ਹੈ ਦਾਰੂ ਭੀ ਇਸੁ ਮਾਹਿ (ਵਾਰ ਆਸਾ)
Ego is a chronic malady and in itself lies the cure (Siri Guru Granth Sahib, Page 466)
In itself it can be useful and even necessary to functioning. But when you act attached to the ego as if it is your real nature and as if it defines your reality and scope, then you create pain, unhappiness and problems. Your spiritual reality is that you are part of a vast creation. In experience, you are a creature that is not separate from the larger matrix of life and consciousness. When you act in your specific role toward a goal or project, you must focus and limit your attention and energies. You can do that and forget anything other than your current attachment. Or you can act with passion and commitment as you remember and intuitively connect to the larger Self that is you and to the larger world that you act within.
The ego creates a kind of forgetfulness. You forget the Creator.
ਕੂਡ਼ਿ ਕੂਡ਼ੈ ਨੇਹੁ ਲਗਾ ਵਿਸਰਿਆ ਕਰਤਾਰੁ ।। (ਆਸਾ ਦੀ ਵਾਰ)
Being attached to temporary created items
The Creator is forgotten. (Asa de Vaar, Page 468; Siri Guru Granth Sahib)
You forget vastness. The energy of the mind begins to act in narrow ways with petty feelings and limiting beliefs. You act according to the information patterns of a small part of your potential and experience.
What is the alternative? If we cannot base our identity in the known or on any limied position of ego what can we do? Act egolessly. Base an identity in the Infinite and the connection to your sense of the Infinite. That may seem a great or mystic accomplishment, but it is the natural state of the Self. We are part of the Infinite by nature. It is the mind that forms a limited sense of self. We do not need to create an Infinite self. It always exists. That existence is called SAT NAAM - SAT is reality, truth or existence. NAAM is the identity of creative name. We are all in essence a reality that vibrates and creates. Most of the time we act as if we are not. That is the pain imposed on our life by ego. The Shabad removes that pain by removing barriers to perceiving and acting from the Real Self.