OBP stands for on base percentage or average and basically it measures how often a player reaches a base. This is the formula: OBP = (Hits + Walks + Hit by Pitch) / (At Bats + Walks + Hit by Pitch + Sacrifice Flies)
Good players tend to have an OBP of .370
ERA stands for earned run average and is the average number of runs given up by a pitcher per 9 innings. (Total # earned runs allowed) / (innings pitched) x 9
A good ERA would be 3.00, but I'm not sure if a lot of pitchers have an ERA close to that right now.
An inning is basically 6 outs. Each team gets 3 and the inning is divided into top and bottom halves, with the home team pitching in the top and batting in the 2nd half. When a pitcher pitches 2/3 of an inning, that means they got 2 outs before they were replaced with another pitcher.
There are a lot of stats in baseball and it's hard to remember what they all mean sometimes.