Many would say that the light Jesus speaks of in the Gospel is our God-given talents. I think this is certainly valid, and I would never discourage us thinking this way or acting upon it. But I think perhaps the is another, more essential meaning.
In Mark’s version of this saying, we hear Jesus pairing it with the phrase: "The measure with which you measure will be measured out to you." This begins to remind us of the forgiveness we speak of in the Our Father: “forgive us our trespasses as we forgive those who trespass against us.” This begs for us to consider forgiveness as the light that we might shine on others.
Through our forgiveness, everything will come to light. When we are non-judgmental, people will open up to us about struggles, difficulties and even their sins. We see this concept proven out in the confessional. When people know they are going to be forgiven, and not judged, they will come and bring all things to light. We should be known as purveyors of this kind of light. This is the way of God's love. He loves with mercy, with 'steadfast loving-kindness'. What makes it steadfast is that it is not 'conditioned' upon our good behavior. Literally thank God for that. So let your light shine.