There are instructions over here.
The Inbox is where new, potentially interested students will be. They'll come in through typeform surveys. The surveys in the inbox represent a wide range of interest level from students. Some will have just wanted a free giveaway, some really want to get involved with Cru. The goal is to quickly go through the Inbox to find out who is unresponsive or not interested.
Once a student has expressed some interest (or openness to Cru/hearing from you), then you'll add them to your contacts. Contacts are more permanent than Inbox items. Here's a helpful analogy- in the fall, you might text with 50 students at a time. But you don't save their phone numbers and add them to your phone contacts until a certain point (where they've expressed interest, are responding, considering meeting with you, etc.). The Inbox/Contacts pages follow a very similar pattern.
Here are a few ways surveys and contacts get moved around. One of these could have happened (by you accidentally or by an admin).
A survey could get reassigned to someone else or the queue (essentially, the movement's inbox).
After 6 days of inactivity on an Inbox item, the system automatically puts the item back in the queue.
A survey could be in the "My Completed Inbox" list on your Inbox page. This happens when a survey is marked as unresponsive or not interested.
A survey could be created into a Contact from the Inbox (either directly or via status change).
A contact could be reassigned to someone else.
The easiest way is to use the global search at the top of Connections. Click th magnifying glass and start typing a name and you will likely find them there.