Electrical components like batteries and solar panels can be used independently or combined. If combined, they are connected in series or in parallel.1)
Series connections double the voltage while the current (or capacity in Ah when dealing with batteries) remains unchanged.
Parallel connections double the current while the voltage remains unchanged.
common example: 2 12v, 90Ah batteries in parallel = a 12v, 180Ah battery bank. This is not a recommended configuration. It is not due to any shortcoming of parallel connections; 12v "deep cycle" or marine batteries
are rarely real deep cycle batteries and suffer from the treatment van life dishes out.
Wide parallel configurations (like 4S) are difficult to charge/discharge evenly without exotic cable configurations.
When discussing multi-battery banks it is common to describe the electrical layout with a particular notation.
p = parallel
s = series
1p1s - a single battery
1p2s - two batteries in series, as one finds in the common 2x6v GC battery setup
2p1s - two batteries in parallel
2p2s - two sets of batteries in parallel linked in series as one finds in higher-capacity GC-based setups