Going by the right hand column, it lokks to me as though he enlisted (probably as airman 3rd class) on 3 Aug 1917 and was appointed airman second class on 2 Jan 1918. Both of these would have been RFC or RNAS. On 1st April 1918 he was transferred to the RAF on it's formation.
Going by the left hand column it looks as though he attended Flight School (I assume that is what Fg Ssh means). BEF would normally refer to British Expeditionary Force (ie the forces in France). H EST is I think "Home Establishment"
You probably worked all that out already. Others probably have more expert knowledge than I though. What is certain is that he is transferred onto the rolls of the RAF on the day the RAF was officially formed from the RFC and RNAS.