At first glance many Roman coins legends make no sense at all but with a bit of training it becomes second nature. I hunted for a guide to all the abbreviations and mintmarks when I started so I thought that now I have a spare hour or two I'd put one up.
AVG = Augustus
C or CAES = Caesar, often used as title by heir
COS = Consul
D N = Dominus Noster, 'Our Lord' (replaces IMP)
DD NN = Dominorum Nostrorum, the plural of Dominus Noster
III VIR/IIII VIR = Triumvir/Quattuorvir, 'One of Three/Four Men'.
PERP or PP = Perpetuus, 'Continuous' often applied to offices, eg. Censor
P F = Pius Felix,'Dutiful & Happy" (Pious + fortunate etc)
P M = Pontifex Maximus
P P = Pater Patriae "Father of the Nation"
S C = Senatus Consulto, 'by Decree of the Senate'
S P Q R = Senatus Populusque Romanus, 'The Roman Senate and People'.
TR P = Tribunicia Potestas, 'Tribunician Power'.
Alexandria: SMALa / ALEa
Antioch: SMANTa / ANa / ANTa
Aquileia: AQp / SMAQp
Arelate / Constantina : pARL / ARLa / pCONST / pCON
Constantinople: CONSa / CONS
Heraclea: SMHa / TESa / TSa
London: pLN / pLON
Lugdunum: pLG / LVGp
Mediolanum: MD / MDPS / MED
Nicomedia: SMNa / MNa
Ravenna RV / RVPS
Rome: Rp / SMRp
Siscia: aSIS / SISC
Thessalonika: SMTSa / TESa / TSa
Treveri: SMTR / pTR / TRp
All small case letters are variable as they denote a particular workshop within a mint.
Hopefully this helps you identify and understand your coins a bit better.