The solenoid and the solenoid contacts need cleaning.......
Often if you keep trying with the key start a "good enough" solenoid contact enough for starter current can be made.....you may then be OK for a while or the next day have a repeat until you refurbish the solenoid.
This does happen more often with a starter/solenoid that has not been used for a while.
Push the car out of your driveway and have a couple of guys then push the car forward and you pop the clutch in second gear with key on.