Breadboard holes are sized for nice press-fit with 22ga solid wire (the kind we use for most stuff). The rows of holes are connected to each other inside, separated by a center channel. This allows for insertion of IC chips whose pins need to be isolated.
The green highlighting represents holes connected to each other. Note that each row is separated, and rows on the left are separated from rows on the right as well.
The long columns of holes on the sides are also connected, conventionally used for power (red) and ground (black or blue). These are sometimes called busses or rails. The columns are not connected to the columns on the opposite side, so power and ground are often bridged with wires. This also affords the possibility of having different voltages, e.g. 5V on one side for logic, 12V on the other for motors, etc.
The numbers on the rows and letters on the columns are for your reference only – they are all the same inside. You can plug in a wire to any dot on any row, and plug in another wire on the same row (and same side) and they will be connected to each other.
NOTE: On some long breadboards, the busses on the sides are not connected, but split in the middle. This will be indicated by a gap on the breadboard.
Breadboard circuits get messy fast. It’s okay to move quickly as you prototype, but whenever you can take a pause go back and shorten wires and line up nice and straight. This will take more time but will eventually save you time in debugging!