Maturation is term used to describe the maturing process that takes place during the first few months of an aquariums life.
To function as a stable environment an aquarium must contain many forms of bacteria and microscopic life that perform the function of breaking down waste materials, recycling nutrients and converting harmful substances. The time needed for these organisms to grow and form a stable population is the maturation period.
During the maturation period, the aquarium may be prone to bad water conditions if the production of waste exceeds the ability of the organisms in the aquarium to process those waste products. This can happen if the aquarium is stocked with fish too quickly, overfed, or the aquarium filter is miss-managed. The symptoms created by such problems during the maturation period are often described as 'new tank syndrome'.
There are a number of products designed as 'bacterial starters' that help the maturation process by adding bacteria to the aquarium to settle and process waste products. These products should be used with care though, as some can simply introduce more waste, which encourages organisms to grow, but also decreases water quality to potentially dangerous levels.
In many cases it is best to only use these products before fish are added to the aquarium, during this time the water conditions can be monitored with test kits, and fish can be added once the water is deemed safe.
Many fish are sold on the basis as only being suitable for a 'mature' aquarium; this is because they may be sensitive to the water quality fluctuations that are probable in a new aquarium. An aquarium would normally be considered matured when it contains a significant population of fish and has been running for at least 4-6 months.