Acclimate is the process of introducing your fish slowly to the tank water so you can avoid harsh parameter changes that the fish can't adapt to, which results in them going into shock and often kills them.
Generally, the accepted procedure is to get a container and put the fish along with the store water in it. Slowly add tank water over time and when the container gets too full, empty a portion of the water. After a while, you have fish sitting in tank water and then you can add them to your community.
I generally take anywhere from 20 to 30 minutes. For more sensitive fish, doing it over 45 minutes to an hour can be recommended.