To continuously supply a load of up to 30-V.A at the rated secondary voltage of the transformer.
I'm assuming these are RMS values. Power is equivalent to the voltage times the current: 30VA = 24*I I = 1.25A. If the load is 30VA, you should put in a fuse larger than 1.25A. If the transformer this is coming from is rated at 30VA, and the load is below this (say typically 20 watts resistive load), you may want to undersize the fuse to insure the transformer is protected.