Colonization was the creation of settlements in areas where civilization had not previously existed.
The colonization of a planet around Alpha Centauri (the Jupiter Mission) had the dual purpose of relieving pressure on Earth's overcrowded 1997 conditions and opening up new fields of resources and scientific exploration. The Jupiter Mission was to be the initial stage of landing and a test run for slowly accelerated colonization. ("The Reluctant Stowaway")
Alpha Control continued to maintain its 'lightships' to replenish future colonization craft. ("The Haunted Lighthouse").
In "No Place to Hide" and "The Reluctant Stowaway", the announcer claims the Robinsons are the first of what may 'be as many as 10 million families per year' setting out to colonize Alpha Centauri's system.