It means the second reserve of the 5th batallion of the Buckinghamshire Regiment.
You might want to look here:
http://www.bayonetstrength.150m.com/stuff/brit ish_regimental_system.htm
where it says
"The Great War placed the most enormous strain on the British Regimental system yet seen. The Army would expand to undreamt of levels in terms of manpower and formations. Millions of men were mobilised over the next four years of conflict and slaughter, yet during that time the only new Regiment raised was the Welsh Guards in 1916. While the Continental armies grew to gargantuan proportions, taking their Regimental numbers into the hundreds, the British Army retained its previous system. Most Regiments were to provide around a dozen Battalions, the equivalent of a full British Division of three 'square' Brigades. The incorporation of Territorial Battalions into the regular Army helped spawn one of the most confusing aspects of the Regimental system. Where a regular Regiment had two Battalions, it would often be affiliated to a Territorial Army Battalion. The New Army Battalions raised from around 1915 took the next number after the TA Battalion number. Territorial Battalions also raised reserve Battalions. When such a Battalion was activated, rather than it taking the next number in the sequence, it took a fractional number of its Territorial predecessor, such a 1/4th Battalion, the next Territorial Reserve being numbered 2/4th and so on. Dependent upon what number the initial Territorial Battalion had borne, units such as the 4/5th Battalion could be found. It was a somewhat confusing system, and one I do not claim to understand fully I have to admit."