A contact in Salesforce is a record about a single individual person and fundamentally it contains information about how to 'contact' a person.
Salesforce Lookup field
A lookup field in Salesforce is used to find other records in Salesforce to build relationships between records. For example, in a contact record one uses a lookup field to find and relate another record such as an organization or community to the contact.
Lookup fields are not searchable in the global search on the home page, but can be searched with the Enhance Lookup feature. See a video about this in knowledge base article Wildcard search and search limitations (video)