This refers to an uninfected person being in close contact, which is within two meters or less for more than 15 minutes in a closed space, with an infected person. In the context of COVID-19, a person is infectious from two days before to 10 days after their symptoms begin.
In the context of COVID-19, the degree of the closeness of contact between an infected person and someone who is susceptible to infection is crucial to establish how fast the virus spreads.
Close contact is also a term used by health agencies to establish testing and tracing regimes. In some countries, a person will be asked to quarantine themselves if they have been in close contact with someone who has tested positive for infection.