A magnetic stripe card stores data in an iron-oxide track that a reader swipes past a head. It is inexpensive and familiar, but the data is easy to read and copy, and the stripe wears out or is erased by magnets and heat. Most new access systems use contactless smart cards instead.
Magnetic stripe is still common on campus and membership cards, usually where the card must also serve a payment or library system.
Why it is considered weak
A stripe holds a static identifier that a reader can capture and an inexpensive writer can copy. There is no cryptographic challenge between the card and the reader.
Where it still makes sense
Low-risk interior doors, or a site already issuing stripe cards for another function, can justify it. Pairing with a PIN raises the bar without replacing the estate.