Attestation is the process of swearing allegiance to the monarch and being formally accepted into her service as a member of the armed forces. It's when your vague interest in joining the military gets serious.
From the Royal Family website:
On enlistment, the [Army, etc.] Acts require members of the Army, Air Force and Royal Marines (who operate ashore under the Army Act) to take an oath of allegiance to the monarch as Head of the Armed Forces (those for whom it is against their religion to take oaths and those who are of no religion, affirm instead of swearing an oath). The Royal Navy was formed hundreds of years ago, and its existence stems from the sovereign's prerogative - members of the Navy have never therefore been required to take the oath.
The oath of allegiance is sworn to the monarch, rather than to Parliament, which might be confused with the political party in power at the time. This reaffirmation of loyalty to the monarch, as Head of State, also ensures that the loyalty of servicemen and women as serving members of the Armed Services (regardless of their personal political beliefs) is not given to any one political party, but to the country in the form of the Head of State.
0306. The British Army must be structured and trained to fight, not for the convenience of administration in peace. On joining the Army soldiers accept an open-ended commitment to serve whenever and wherever they are needed, whatever the difficulties or dangers may be. Such commitment imposes certain limitations on individual freedom, and requires a degree of self-sacrifice. Ultimately it may require soldiers to lay down their lives. Implicitly it requires those in positions of authority to discharge in full their responsibilities and their duty of care to subordinates. Selfless commitment is reflected in the wording of the Oath of Allegiance which is taken on attestation, and in which soldiers agree to subordinate their own interests to those of the unit, Army and Nation, as represented by the Crown:
"I SWEAR BY ALMIGHTY GOD THAT I WILL BE FAITHFUL AND BEAR TRUE ALLEGIANCE TO HER MAJESTY QUEEN ELIZABETH II, HER HEIRS AND SUCCESSORS AND THAT I WILL AS IN DUTY BOUND HONESTLY AND FAITHFULLY DEFEND HER MAJESTY, HER HEIRS AND SUCCESSORS IN PERSON, CROWN AND DIGNITY AGAINST ALL ENEMIES AND WILL OBSERVE AND OBEY ALL ORDERS OF HER MAJESTY, HER HEIRS AND SUCCESSORS AND OF THE GENERALS AND OFFICERS SET OVER ME"
Note: (Those who do not believe in God "SOLEMNLY, SINCERELY AND TRULY DECLARE AND AFFIRM")
0307. Irrespective of private beliefs, this Oath embodies the context within which the British Army fights and operates. It expresses the formal yet personal loyalty of every soldier in the British Army to the Sovereign as head of state. This focus on the Sovereign means that whatever the political views of individual soldiers, the British Army is essentially apolitical. Similarly, the Sovereign is the authority for the Commissions and Warrants of the various categories of officers. There is a similar formal yet personal relationship of Corps and Regiments to the Sovereign. These relationships find expression in the Colours, Standards and other emblems of Regimental and Corps spirit which derive from the Sovereign, and in the Ceremonial which demonstrates publicly the role of the Army in the fabric of the nation.