The openings books doesn't really try to tell you what you should play, it mainly lists lines that have been played and seem to be reasonable. There'll be situations where you have more than one reasonable move, and the choice will be yours.
In the above table, the first dotted line means that both 2. g3 and 2. d3 are common and reasonable moves for while. The second shows that, in response to 2. g3, 2...Nf6, 2...c6, and 2...c5 are common and reasonable moves for Black. Basically the dotted line shows that the table is giving variations from the previous move.
The blank spots in column 5 are there because the variations in columns 4 and 5 don't diverge until move 4 for Black. So variation 4 would go 1. Nf3 d5 2. g3 c5 3. Bg2 Nc6 4. O-O e6 and then continue down column 4, and variation 5 would go 1. Nf3 d5 2. g3 c5 3. Bg2 Nc6 4. O-O e5, differing only in the last move, and then continuing down column 5.
Hello, I've just started reading on openings, but the book doesn't contain any info on how to understand the tables. Here's an example: