The term "insertion", at least in modern military jargon, refers to the method in which operatives (usually covert) are deployed to the battle field.
For example, special forces can utilize High Altitude Low Opening (HALO) jumps to get behind enemy lines. Taskforce Dagger, one of the first units with boots on the ground in Afghanistan, used personnel disguised as locals and good old-fashioned horses to get to some of the more remote locations in country. SEALS are trained in water-based insertion using advanced rebreathers and thermal gear.
While I know little of how spies actually work (and seeing as most of what you see on TV is highly glorified), any job that involves getting into somewhere you're not allowed/supposed to be qualifies as an insertion; the goal of the op could be anything, as suggested by others, but the method of which you gain entry will be the insertion. Planting false evidence, obtaining proof of something or another, setting up an extraction, and any number of other options.