I assume that would be electricity generated by solar panels or a nuclear reactor.
“What produces the electromagnetic waves?”
Although you could use solar cells close to a star, nukes are the real powerhouses.
Once you get outside the asteroid belt, solar cells don’t get much of a jolt. For Earth to Moon or orbital operations, they could be workhorses.
Nuclear reactors could give many orders of magnitude greater thrust to weight ratios than rocket fuel (or solar cells, in almost any scenario).
For example, a reactor the size of two five gallon buckets, could produce megawatts of electricity for ten years with exceptional reliability.