Containment is a term that refers to the tight control of substances, animals, infections, outbreaks, or people. It is aimed at not allowing the spread of a wide range of conditions and things: disease, infection, populations, political ideas. This is accomplished by multiple means depending on the substances involved and the situation. There can be different levels of containment.
For example, when dealing with infectious or dangerous bioorganisms this process is called biocontainment. Primary containment is basic safety procedures such as using effective, safe storage measures along with protective gear. Imagine some researchers at a biological research center may have been unintentionally exposed to a spreadable virus. Containment would be accomplished by making sure all of the safety measures were intact, decontaminate those who may have been affected, and possibly being quarantined and monitored until enough time had passed so that they could be deemed safe.