The phrase "national security" contains the various implements utilized by the state in order to ensure the tranquility of its borders, the independence of its economy and the security of its residents (that is, their personal security). Once the basic phrase is understood, one can understand just how important the phrase is in a country such as Israel -- or, more accurately, in the State of Israel itself.
"The security establishment," the general phrase assigned by the State of Israel to its national security organization, contains a number of bodies: the military -- the IDF (Israel Defense Forces), responsible for the security of the country's borders, "BaT"Sh" ("ongoing security" activity) and an aerial border security system (through a branch known, of course, as the Israeli Air Force). The military is headed by the General Staff (which in, turn, headed by the Chief of General Staff), which reports directly to the Minister of Defense and to the various Knesset (parliament) committees (as required). Intelligence service -- the Mossad, responsible for the collection and dissemination of information around the world, for joint cooperation with parallel intelligence-gathering bodies, for the recruitment of foreign agents, and more. The Mossad answers directly to the Prime Minister's Office (and is frequently referred to as "the Prime Minister's Office"). Internal security body -- the ShaBa"K ("General Security Service"), responsible for such internal security matters as organized crime and underground movements.
Other bodies exist in order to complement the work of the groups mentioned above. For instance, the National Security Council (established during Benjamin Netanyahu's term as prime minister and leader of the Likud faction) is responsible for matters of policy.
The Coordinator of Activities in the Territories is another hybrid body established in order to maintain control over and monitor the happenings in the "Occupied Territories."* Under the Coordinator of Activities there exists a body known as the Coordination and Liaison Headquarters, a body comprised of Israeli officers and Palestinian representatives during the Oslo implementation phase in 1993. It was supposed to serve as a point of interface between the Israelis on one hand and the Palestinians on the other, but when the violent disturbances erupted (the Wall Tunnel Events of 1997) the liaison system collapsed and became, in time, a target of attacks from those opposed to the Peace Process.
The State of Israel faces daily threats, both from without and from within. It is one of the only countries in the world forced to contend with such a complex demographic situation in such a small parcel of land. The security establishment must deal with terror attempts, information gathering, and the protection of the country's borders and airspace on a daily basis, and it engages in those activities both in Israel and abroad.
*I do not intend to express a stance or a critique.