Karma means action, and also the consequences that follow from it. In everyday speech it is often reduced to what goes around comes around. The Gita's treatment is richer. It studies what makes an action what it is, what causes it, what shapes its results and how a person can act without being bound by what follows.
Gita 14.16 says the quality behind an action determines its result: pure action brings clarity, passion-driven action ends in suffering, and action done in darkness produces ignorance. Examine the motive and state of mind behind your choices. Gita 4.12 reflects on how people seeking the fruits of action often choose immediate rewards over deeper aims, and Gita 18.12 says action bears unpleasant, pleasant and mixed results, which do not bind those who have given up the fruits.
The Gita also analyzes causes. Gita 18.13 and 18.14 name five factors behind any action, including the body, the agent, the instruments, the many kinds of effort and a divine element beyond human control, so do your part fully while accepting what lies outside it. Gita 18.15 says that whatever one does through body, speech or mind has these same causes, and Gita 18.60 says past conditioning exerts a strong pull. Study all the factors behind an outcome before crediting or blaming yourself.