It is not confirming that the burner ignited, so it is shutting itself off and running a purge cycle. It might repleat that a few times before it shuts down with an error message. The flame sensor itself might be dirty - cleaning it might help. It could be out of position (unlikely unless you'd been in there cleaning things), the flame could be messed up so it is not actually hitting the sensor, the sensor could be bad and needs to be replaced, or the circuit that 'reads' the flame sensor is either bad, or there's a loose connection getting it there.
Depending on the type of sensor, you may be able to easily test it - it would be described in the maintenance manual. I'd look at it, clean it if it's dirty (maybe some steel wool - depends on the type), and make sure that where it is, the flame actually hits it.