Although not a verifiable effect, the ‘sandbox’ refers to an “experience” had by some where newly submitted websites are “held back” in ranking temporarily by Google while it qualifies the inbound links in an effort to maintain the integrity of its system of page ranking.
Conversely, “others” report the ‘reverse sandbox’ effect, where their newly submitted websites are given a temporary boost in rankings in order to stimulate organic search.
Both are suppositions and neither is verifiable so the best advice is to ensure each one of your web pages is highly focused on the keywords and phrases for which it is being optimized and that all ‘inbound links’ support the objectives of your campaign. You can enhance your opportunities to rank well in the search engines by helping them to maintain the integrity of their system and by providing high quality content that is easy to index and retrieve.
