What baseload is is electricity supply which is only economical if you use it all the time. Nuclear falls into this category because of its very high capital cost and low op-ex. If it's cheaper than dispatchable power (nuclear isn't) it's nice to have as much of it as the minimum demand that you see on the grid, to lower costs. If it's as expensive, or more expensive, than dispatchable power, that's fine, you just don't need it at all and can replace it entirely with dispatchable power.

It's similar to wind and solar in this, which also aren't dispatchable (though there supply curve looks different than the constant supply curve which "base load" is used to mean). Except wind and solar actually are cheaper than dispatchable power so they make economic sense.

The term is half marketing term and half a theory that constant supply non-dispatchable power would be significantly cheaper than dispatchable power so we should organize the grid around it. That theory didn't really pan out (apart from some places with non-storable hydro, and a few with geothermal).

basically, base load means the lowest point of demand on the grid. And you matched that with slow-to-respond thermal power plants (coal mainly, also nukes). Because those are slow to respond and are most profitable running at 100%, so you tried to keep them there. So called base load generation.

But note there is no rule of the universe that says you have to meet the base load demand with some static constant power source, you can get it from anywhere. And now, since renewables and batteries are cheaper than this base load generation, it knocks them off the grid rendering it unprofitable. So the whole concept of base load supply is obsolete. Anyway, the linked blog explains it better.