A magnetic strip based credit card is one where the information about the card is stored on a magnetic strip on the card.
This is the black strip that is present behind the card.
This magnetic strip is passed through a reading machine which decodes the information.
This is why one has to be very careful that the details on the magnetic strip are not captured by someone who wishes to misuse the information.
This is an older technology but still many credit cards across the world still has this in use,